docs: Add section about faster cache loading (#2932)

Co-authored-by: Maria Shodunke <maria-robobug@users.noreply.github.com>

.github/workflows/build-clio-docker-image.yml

@@ -48,7 +48,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Download Clio binary from artifact
         if: ${{ inputs.artifact_name != null }}

.github/workflows/build.yml

@@ -23,6 +23,7 @@ on:
       - "cmake/**"
       - "src/**"
       - "tests/**"
+      - "benchmarks/**"
 
       - docs/config-description.md
   workflow_dispatch:
@@ -100,7 +101,7 @@ jobs:
       image: ghcr.io/xrplf/clio-ci:14342e087ceb8b593027198bf9ef06a43833c696
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0
         with:

.github/workflows/check-libxrpl.yml

@@ -24,12 +24,12 @@ jobs:
       image: ghcr.io/xrplf/clio-ci:14342e087ceb8b593027198bf9ef06a43833c696
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: false
 
@@ -92,7 +92,7 @@ jobs:
       issues: write
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Create an issue
         uses: ./.github/actions/create-issue

.github/workflows/clang-tidy.yml

@@ -39,12 +39,12 @@ jobs:
       pull-requests: write
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: false
 
@@ -107,7 +107,7 @@ jobs:
 
       - name: Create PR with fixes
         if: ${{ steps.files_changed.outcome != 'success' && github.event_name != 'pull_request' }}
-        uses: peter-evans/create-pull-request@98357b18bf14b5342f975ff684046ec3b2a07725 # v8.0.0
+        uses: peter-evans/create-pull-request@c0f553fe549906ede9cf27b5156039d195d2ece0 # v8.1.0
         env:
           GH_REPO: ${{ github.repository }}
           GH_TOKEN: ${{ github.token }}

.github/workflows/docs.yml

@@ -22,12 +22,12 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           lfs: true
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: false
 

.github/workflows/nightly.yml

@@ -169,7 +169,7 @@ jobs:
       issues: write
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Create an issue
         uses: ./.github/actions/create-issue

.github/workflows/pre-commit.yml

@@ -8,7 +8,7 @@ on:
 
 jobs:
   run-hooks:
-    uses: XRPLF/actions/.github/workflows/pre-commit.yml@282890f46d6921249d5659dd38babcb0bd8aef48
+    uses: XRPLF/actions/.github/workflows/pre-commit.yml@01163508e81d7dd63d4601d4090b297a260b18c2
     with:
       runs_on: heavy
       container: '{ "image": "ghcr.io/xrplf/clio-pre-commit:14342e087ceb8b593027198bf9ef06a43833c696" }'

.github/workflows/reusable-build.yml

@@ -90,12 +90,12 @@ jobs:
         if: ${{ runner.os == 'macOS' }}
         uses: XRPLF/actions/cleanup-workspace@cf0433aa74563aead044a1e395610c96d65a37cf
 
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: ${{ inputs.download_ccache }}
 
@@ -113,7 +113,7 @@ jobs:
 
       - name: Restore ccache cache
         if: ${{ inputs.download_ccache && github.ref != 'refs/heads/develop' }}
-        uses: actions/cache/restore@9255dc7a253b0ccc959486e2bca901246202afeb # v5.0.1
+        uses: actions/cache/restore@8b402f58fbc84540c8b491a91e594a4576fec3d7 # v5.0.2
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ steps.cache_key.outputs.key }}
@@ -164,7 +164,7 @@ jobs:
 
       - name: Save ccache cache
         if: ${{ inputs.upload_ccache && github.ref == 'refs/heads/develop' }}
-        uses: actions/cache/save@9255dc7a253b0ccc959486e2bca901246202afeb # v5.0.1
+        uses: actions/cache/save@8b402f58fbc84540c8b491a91e594a4576fec3d7 # v5.0.2
         with:
           path: ${{ env.CCACHE_DIR }}
           key: ${{ steps.cache_key.outputs.key }}

.github/workflows/reusable-release.yml

@@ -55,12 +55,12 @@ jobs:
       contents: write
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: false
 

.github/workflows/reusable-test.yml

@@ -54,7 +54,7 @@ jobs:
         if: ${{ runner.os == 'macOS' }}
         uses: XRPLF/actions/cleanup-workspace@cf0433aa74563aead044a1e395610c96d65a37cf
 
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 
@@ -126,17 +126,17 @@ jobs:
         if: ${{ runner.os == 'macOS' }}
         uses: XRPLF/actions/cleanup-workspace@cf0433aa74563aead044a1e395610c96d65a37cf
 
-      - name: Restart colima
+      - name: Delete and start colima (macOS)
+        # This is a temporary workaround for colima issues on macOS runners
         if: ${{ runner.os == 'macOS' }}
         run: |
           colima delete --force
           colima start
 
-      - name: Spin up scylladb
+      - name: Spin up scylladb (macOS)
         if: ${{ runner.os == 'macOS' }}
-        timeout-minutes: 3
+        timeout-minutes: 1
         run: |
-          docker rm --force scylladb || true
           docker run \
             --detach \
             --name scylladb \
@@ -148,8 +148,12 @@ jobs:
             --memory 16G \
             scylladb/scylla
 
+      - name: Wait for scylladb container to be healthy (macOS)
+        if: ${{ runner.os == 'macOS' }}
+        timeout-minutes: 1
+        run: |
           until [ "$(docker inspect -f '{{.State.Health.Status}}' scylladb)" == "healthy" ]; do
-            sleep 5
+            sleep 1
           done
 
       - uses: actions/download-artifact@37930b1c2abaa49bbe596cd826c3c89aef350131 # v7.0.0

.github/workflows/reusable-upload-coverage-report.yml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
 

.github/workflows/update-docker-ci.yml

@@ -56,7 +56,7 @@ jobs:
     needs: repo
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -94,7 +94,7 @@ jobs:
     needs: repo
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -132,7 +132,7 @@ jobs:
     needs: [repo, gcc-amd64, gcc-arm64]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -183,7 +183,7 @@ jobs:
     needs: repo
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -219,7 +219,7 @@ jobs:
     needs: [repo, gcc-merge]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -250,7 +250,7 @@ jobs:
     needs: [repo, gcc-merge]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -281,7 +281,7 @@ jobs:
     needs: [repo, tools-amd64, tools-arm64]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Get changed files
         id: changed-files
@@ -316,7 +316,7 @@ jobs:
     needs: [repo, tools-merge]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: ./.github/actions/build-docker-image
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -338,7 +338,7 @@ jobs:
     needs: [repo, gcc-merge, clang, tools-merge]
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: ./.github/actions/build-docker-image
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/upload-conan-deps.yml

@@ -52,7 +52,7 @@ jobs:
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Calculate conan matrix
         id: set-matrix
@@ -75,10 +75,10 @@ jobs:
       CONAN_PROFILE: ${{ matrix.compiler }}${{ matrix.sanitizer_ext }}
 
     steps:
-      - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
       - name: Prepare runner
-        uses: XRPLF/actions/prepare-runner@f05cab7b8541eee6473aa42beb9d2fe35608a190
+        uses: XRPLF/actions/prepare-runner@e8d2d2a546a03e1d161dca52890705f3bc641215
         with:
           enable_ccache: false
 

.gitignore

@@ -4,6 +4,7 @@
 .build
 .cache
 .vscode
+.zed
 .python-version
 .DS_Store
 .sanitizer-report

benchmarks/CMakeLists.txt

@@ -16,5 +16,5 @@ target_sources(
 include(deps/gbench)
 
 target_include_directories(clio_benchmark PRIVATE .)
-target_link_libraries(clio_benchmark PUBLIC clio_util clio_rpc benchmark::benchmark_main spdlog::spdlog)
+target_link_libraries(clio_benchmark PRIVATE clio_rpc clio_util benchmark::benchmark_main spdlog::spdlog)
 set_target_properties(clio_benchmark PROPERTIES RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR})

benchmarks/rpc/WorkQueueBenchmarks.cpp

@@ -29,8 +29,6 @@
 
 #include <benchmark/benchmark.h>
 #include <boost/asio/steady_timer.hpp>
-#include <boost/asio/thread_pool.hpp>
-#include <boost/json/object.hpp>
 
 #include <algorithm>
 #include <atomic>

docs/run-clio.md

@@ -93,3 +93,42 @@ To completely disable Prometheus metrics add `"prometheus": { "enabled": false }
 It is important to know that Clio responds to Prometheus request only if they are admin requests. If you are using the admin password feature, the same password should be provided in the Authorization header of Prometheus requests.
 
 You can find an example Docker Compose file, with Prometheus and Grafana configs, in [examples/infrastructure](../docs/examples/infrastructure/).
+
+## Ledger cache file
+
+Since version 2.7.0, Clio supports saving the ledger cache to a local file on shutdown and loading it on startup. This feature is disabled by default but can significantly improve restart times.
+
+### Benefits
+
+- **Faster startup**: Loading cache from a file takes less than a minute, compared to 40-90 minutes on Mainnet when loading from the database.
+- **Reduced database load**: Clio doesn't put extra load on the database when starting with a cache file.
+- **Improved availability**: Faster restart times mean less downtime during maintenance or updates.
+
+> [!NOTE]
+> This feature only works when Clio is restarted. When starting Clio for the first time, the cache must be loaded from `rippled` or the database as usual.
+
+### Configuration
+
+To enable the ledger cache file feature, specify the [`cache.file.path`](./config-description.md#cachefilepath) option in your `config.json`:
+
+```json
+"cache": {
+    "file": {
+        "path": "/path/to/cache/file"
+    }
+}
+```
+
+You can optionally configure additional settings such as [`cache.file.max_sequence_age`](./config-description.md#cachefilemax_sequence_age) and [`cache.file.async_save`](./config-description.md#cachefileasync_save) to fine-tune the behavior. For a complete list of available options and their default values, see the [Configuration Description](./config-description.md#cachefilepath) documentation.
+
+### How it works
+
+1. **On shutdown**: Clio saves the current ledger cache to the specified file path. The file includes a hash for integrity verification.
+2. **On startup**: Clio checks if a cache file exists at the configured path. If the file exists, Clio will:
+   - Verify the file's integrity to ensure it is complete and not corrupted.
+   - Compare the latest ledger sequence in the cache file with the latest sequence in the database.
+   - Use the cache file only if the difference is less than [`cache.file.max_sequence_age`](./config-description.md#cachefilemax_sequence_age).
+   - If validation fails or the cache is too old, Clio will fall back to loading from the database.
+
+> [!IMPORTANT]
+> The cache file path should point to a location with sufficient disk space. On typical deployments, the cache file size can be several gigabytes.

src/app/ClioApplication.cpp

@@ -29,6 +29,8 @@
 #include "etl/ETLService.hpp"
 #include "etl/LoadBalancer.hpp"
 #include "etl/NetworkValidatedLedgers.hpp"
+#include "etl/SystemState.hpp"
+#include "etl/WriterState.hpp"
 #include "feed/SubscriptionManager.hpp"
 #include "migration/MigrationInspectorFactory.hpp"
 #include "rpc/Counters.hpp"
@@ -121,7 +123,11 @@ ClioApplication::run(bool const useNgWebServer)
     // Interface to the database
     auto backend = data::makeBackend(config_, cache);
 
-    cluster::ClusterCommunicationService clusterCommunicationService{backend};
+    auto systemState = etl::SystemState::makeSystemState(config_);
+
+    cluster::ClusterCommunicationService clusterCommunicationService{
+        backend, std::make_unique<etl::WriterState>(systemState)
+    };
     clusterCommunicationService.run();
 
     auto const amendmentCenter = std::make_shared<data::AmendmentCenter const>(backend);
@@ -151,7 +157,9 @@ ClioApplication::run(bool const useNgWebServer)
     );
 
     // ETL is responsible for writing and publishing to streams. In read-only mode, ETL only publishes
-    auto etl = etl::ETLService::makeETLService(config_, ctx, backend, subscriptions, balancer, ledgers);
+    auto etl = etl::ETLService::makeETLService(
+        config_, std::move(systemState), ctx, backend, subscriptions, balancer, ledgers
+    );
 
     auto workQueue = rpc::WorkQueue::makeWorkQueue(config_);
     auto counters = rpc::Counters::makeCounters(workQueue);
@@ -197,7 +205,16 @@ ClioApplication::run(bool const useNgWebServer)
         }
 
         appStopper_.setOnStop(
-            Stopper::makeOnStopCallback(httpServer.value(), *balancer, *etl, *subscriptions, *backend, cacheSaver, ioc)
+            Stopper::makeOnStopCallback(
+                httpServer.value(),
+                *balancer,
+                *etl,
+                *subscriptions,
+                *backend,
+                cacheSaver,
+                clusterCommunicationService,
+                ioc
+            )
         );
 
         // Blocks until stopped.
@@ -213,7 +230,9 @@ ClioApplication::run(bool const useNgWebServer)
 
     auto const httpServer = web::makeHttpServer(config_, ioc, dosGuard, handler, cache);
     appStopper_.setOnStop(
-        Stopper::makeOnStopCallback(*httpServer, *balancer, *etl, *subscriptions, *backend, cacheSaver, ioc)
+        Stopper::makeOnStopCallback(
+            *httpServer, *balancer, *etl, *subscriptions, *backend, cacheSaver, clusterCommunicationService, ioc
+        )
     );
 
     // Blocks until stopped.

src/app/Stopper.hpp

@@ -19,6 +19,7 @@
 
 #pragma once
 
+#include "cluster/Concepts.hpp"
 #include "data/BackendInterface.hpp"
 #include "data/LedgerCacheSaver.hpp"
 #include "etl/ETLServiceInterface.hpp"
@@ -82,10 +83,14 @@ public:
      * @param subscriptions The subscription manager to stop.
      * @param backend The backend to stop.
      * @param cacheSaver The ledger cache saver
+     * @param clusterCommunicationService The cluster communication service to stop.
      * @param ioc The io_context to stop.
      * @return The callback to be called on application stop.
      */
-    template <web::SomeServer ServerType, data::SomeLedgerCacheSaver LedgerCacheSaverType>
+    template <
+        web::SomeServer ServerType,
+        data::SomeLedgerCacheSaver LedgerCacheSaverType,
+        cluster::SomeClusterCommunicationService ClusterCommunicationServiceType>
     static std::function<void(boost::asio::yield_context)>
     makeOnStopCallback(
         ServerType& server,
@@ -94,6 +99,7 @@ public:
         feed::SubscriptionManagerInterface& subscriptions,
         data::BackendInterface& backend,
         LedgerCacheSaverType& cacheSaver,
+        ClusterCommunicationServiceType& clusterCommunicationService,
         boost::asio::io_context& ioc
     )
     {
@@ -111,6 +117,8 @@ public:
             });
             coroutineGroup.asyncWait(yield);
 
+            clusterCommunicationService.stop();
+
             etl.stop();
             LOG(util::LogService::info()) << "ETL stopped";
 

src/cluster/Backend.cpp

@@ -0,0 +1,134 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/Backend.hpp"
+
+#include "cluster/ClioNode.hpp"
+#include "data/BackendInterface.hpp"
+#include "etl/WriterState.hpp"
+
+#include <boost/asio/spawn.hpp>
+#include <boost/asio/thread_pool.hpp>
+#include <boost/json/parse.hpp>
+#include <boost/json/serialize.hpp>
+#include <boost/json/value.hpp>
+#include <boost/json/value_from.hpp>
+#include <boost/json/value_to.hpp>
+#include <boost/uuid/random_generator.hpp>
+#include <boost/uuid/uuid.hpp>
+#include <fmt/format.h>
+
+#include <chrono>
+#include <memory>
+#include <utility>
+#include <vector>
+
+namespace cluster {
+
+Backend::Backend(
+    boost::asio::thread_pool& ctx,
+    std::shared_ptr<data::BackendInterface> backend,
+    std::unique_ptr<etl::WriterStateInterface const> writerState,
+    std::chrono::steady_clock::duration readInterval,
+    std::chrono::steady_clock::duration writeInterval
+)
+    : backend_(std::move(backend))
+    , writerState_(std::move(writerState))
+    , readerTask_(readInterval, ctx)
+    , writerTask_(writeInterval, ctx)
+    , selfUuid_(std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator{}()))
+{
+}
+
+void
+Backend::run()
+{
+    readerTask_.run([this](boost::asio::yield_context yield) {
+        auto clusterData = doRead(yield);
+        onNewState_(selfUuid_, std::make_shared<ClusterData>(std::move(clusterData)));
+    });
+
+    writerTask_.run([this]() { doWrite(); });
+}
+
+Backend::~Backend()
+{
+    stop();
+}
+
+void
+Backend::stop()
+{
+    readerTask_.stop();
+    writerTask_.stop();
+}
+
+ClioNode::CUuid
+Backend::selfId() const
+{
+    return selfUuid_;
+}
+
+Backend::ClusterData
+Backend::doRead(boost::asio::yield_context yield)
+{
+    BackendInterface::ClioNodesDataFetchResult expectedResult;
+    try {
+        expectedResult = backend_->fetchClioNodesData(yield);
+    } catch (...) {
+        expectedResult = std::unexpected{"Failed to fetch Clio nodes data"};
+    }
+
+    if (!expectedResult.has_value()) {
+        return std::unexpected{std::move(expectedResult).error()};
+    }
+
+    std::vector<ClioNode> otherNodesData;
+    for (auto const& [uuid, nodeDataStr] : expectedResult.value()) {
+        if (uuid == *selfUuid_) {
+            continue;
+        }
+
+        boost::system::error_code errorCode;
+        auto const json = boost::json::parse(nodeDataStr, errorCode);
+        if (errorCode.failed()) {
+            return std::unexpected{fmt::format("Error parsing json from DB: {}", nodeDataStr)};
+        }
+
+        auto expectedNodeData = boost::json::try_value_to<ClioNode>(json);
+        if (expectedNodeData.has_error()) {
+            return std::unexpected{fmt::format("Error converting json to ClioNode: {}", nodeDataStr)};
+        }
+        *expectedNodeData->uuid = uuid;
+        otherNodesData.push_back(std::move(expectedNodeData).value());
+    }
+    otherNodesData.push_back(ClioNode::from(selfUuid_, *writerState_));
+    return otherNodesData;
+}
+
+void
+Backend::doWrite()
+{
+    auto const selfData = ClioNode::from(selfUuid_, *writerState_);
+    boost::json::value jsonValue{};
+    boost::json::value_from(selfData, jsonValue);
+    backend_->writeNodeMessage(*selfData.uuid, boost::json::serialize(jsonValue.as_object()));
+}
+
+}  // namespace cluster

src/cluster/Backend.hpp

@@ -0,0 +1,147 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "cluster/ClioNode.hpp"
+#include "cluster/impl/RepeatedTask.hpp"
+#include "data/BackendInterface.hpp"
+#include "etl/WriterState.hpp"
+#include "util/log/Logger.hpp"
+
+#include <boost/asio/any_io_executor.hpp>
+#include <boost/asio/cancellation_signal.hpp>
+#include <boost/asio/execution_context.hpp>
+#include <boost/asio/executor.hpp>
+#include <boost/asio/spawn.hpp>
+#include <boost/asio/strand.hpp>
+#include <boost/asio/thread_pool.hpp>
+#include <boost/signals2/connection.hpp>
+#include <boost/signals2/signal.hpp>
+#include <boost/signals2/variadic_signal.hpp>
+#include <boost/uuid/uuid.hpp>
+
+#include <chrono>
+#include <concepts>
+#include <memory>
+#include <string>
+#include <vector>
+
+namespace cluster {
+
+/**
+ * @brief Backend communication handler for cluster state synchronization.
+ *
+ * This class manages reading and writing cluster state information to/from the backend database.
+ * It periodically reads the state of other nodes in the cluster and writes the current node's state,
+ * enabling cluster-wide coordination and awareness.
+ */
+class Backend {
+public:
+    /** @brief Type representing cluster data result - either a vector of nodes or an error message */
+    using ClusterData = std::expected<std::vector<ClioNode>, std::string>;
+
+private:
+    util::Logger log_{"ClusterCommunication"};
+
+    std::shared_ptr<data::BackendInterface> backend_;
+    std::unique_ptr<etl::WriterStateInterface const> writerState_;
+
+    impl::RepeatedTask<boost::asio::thread_pool> readerTask_;
+    impl::RepeatedTask<boost::asio::thread_pool> writerTask_;
+
+    ClioNode::Uuid selfUuid_;
+
+    boost::signals2::signal<void(ClioNode::CUuid, std::shared_ptr<ClusterData const>)> onNewState_;
+
+public:
+    /**
+     * @brief Construct a Backend communication handler.
+     *
+     * @param ctx The execution context for asynchronous operations
+     * @param backend Interface to the backend database
+     * @param writerState State indicating whether this node is writing to the database
+     * @param readInterval How often to read cluster state from the backend
+     * @param writeInterval How often to write this node's state to the backend
+     */
+    Backend(
+        boost::asio::thread_pool& ctx,
+        std::shared_ptr<data::BackendInterface> backend,
+        std::unique_ptr<etl::WriterStateInterface const> writerState,
+        std::chrono::steady_clock::duration readInterval,
+        std::chrono::steady_clock::duration writeInterval
+    );
+
+    ~Backend();
+
+    Backend(Backend&&) = delete;
+    Backend&
+    operator=(Backend&&) = delete;
+    Backend(Backend const&) = delete;
+    Backend&
+    operator=(Backend const&) = delete;
+
+    /**
+     * @brief Start the backend read and write tasks.
+     *
+     * Begins periodic reading of cluster state from the backend and writing of this node's state.
+     */
+    void
+    run();
+
+    /**
+     * @brief Stop the backend read and write tasks.
+     *
+     * Stops all periodic tasks and waits for them to complete.
+     */
+    void
+    stop();
+
+    /**
+     * @brief Subscribe to new cluster state notifications.
+     *
+     * @tparam S Callable type accepting (ClioNode::cUUID, ClusterData)
+     * @param s Subscriber callback to be invoked when new cluster state is available
+     * @return A connection object that can be used to unsubscribe
+     */
+    template <typename S>
+        requires std::invocable<S, ClioNode::CUuid, std::shared_ptr<ClusterData const>>
+    boost::signals2::connection
+    subscribeToNewState(S&& s)
+    {
+        return onNewState_.connect(s);
+    }
+
+    /**
+     * @brief Get the UUID of this node in the cluster.
+     *
+     * @return The UUID of this node.
+     */
+    ClioNode::CUuid
+    selfId() const;
+
+private:
+    ClusterData
+    doRead(boost::asio::yield_context yield);
+
+    void
+    doWrite();
+};
+
+}  // namespace cluster

src/cluster/CMakeLists.txt

@@ -1,5 +1,7 @@
 add_library(clio_cluster)
 
-target_sources(clio_cluster PRIVATE ClioNode.cpp ClusterCommunicationService.cpp)
+target_sources(
+  clio_cluster PRIVATE Backend.cpp ClioNode.cpp ClusterCommunicationService.cpp Metrics.cpp WriterDecider.cpp
+)
 
 target_link_libraries(clio_cluster PRIVATE clio_util clio_data)

src/cluster/ClioNode.cpp

@@ -19,6 +19,7 @@
 
 #include "cluster/ClioNode.hpp"
 
+#include "etl/WriterState.hpp"
 #include "util/TimeUtils.hpp"
 
 #include <boost/json/conversion.hpp>
@@ -26,39 +27,72 @@
 #include <boost/json/value.hpp>
 #include <boost/uuid/uuid.hpp>
 
+#include <chrono>
+#include <cstdint>
 #include <memory>
 #include <stdexcept>
 #include <string>
 #include <string_view>
+#include <utility>
 
 namespace cluster {
 
 namespace {
 
-struct Fields {
+struct JsonFields {
     static constexpr std::string_view const kUPDATE_TIME = "update_time";
+    static constexpr std::string_view const kDB_ROLE = "db_role";
 };
 
 }  // namespace
 
+ClioNode
+ClioNode::from(ClioNode::Uuid uuid, etl::WriterStateInterface const& writerState)
+{
+    auto const dbRole = [&writerState]() {
+        if (writerState.isReadOnly()) {
+            return ClioNode::DbRole::ReadOnly;
+        }
+        if (writerState.isFallback()) {
+            return ClioNode::DbRole::Fallback;
+        }
+        if (writerState.isLoadingCache()) {
+            return ClioNode::DbRole::LoadingCache;
+        }
+
+        return writerState.isWriting() ? ClioNode::DbRole::Writer : ClioNode::DbRole::NotWriter;
+    }();
+    return ClioNode{.uuid = std::move(uuid), .updateTime = std::chrono::system_clock::now(), .dbRole = dbRole};
+}
+
 void
 tag_invoke(boost::json::value_from_tag, boost::json::value& jv, ClioNode const& node)
 {
     jv = {
-        {Fields::kUPDATE_TIME, util::systemTpToUtcStr(node.updateTime, ClioNode::kTIME_FORMAT)},
+        {JsonFields::kUPDATE_TIME, util::systemTpToUtcStr(node.updateTime, ClioNode::kTIME_FORMAT)},
+        {JsonFields::kDB_ROLE, static_cast<int64_t>(node.dbRole)}
     };
 }
 
 ClioNode
 tag_invoke(boost::json::value_to_tag<ClioNode>, boost::json::value const& jv)
 {
-    auto const& updateTimeStr = jv.as_object().at(Fields::kUPDATE_TIME).as_string();
+    auto const& updateTimeStr = jv.as_object().at(JsonFields::kUPDATE_TIME).as_string();
     auto const updateTime = util::systemTpFromUtcStr(std::string(updateTimeStr), ClioNode::kTIME_FORMAT);
     if (!updateTime.has_value()) {
         throw std::runtime_error("Failed to parse update time");
     }
 
-    return ClioNode{.uuid = std::make_shared<boost::uuids::uuid>(), .updateTime = updateTime.value()};
+    auto const dbRoleValue = jv.as_object().at(JsonFields::kDB_ROLE).as_int64();
+    if (dbRoleValue > static_cast<int64_t>(ClioNode::DbRole::MAX))
+        throw std::runtime_error("Invalid db_role value");
+
+    return ClioNode{
+        // Json data doesn't contain uuid so leaving it empty here. It will be filled outside of this parsing
+        .uuid = std::make_shared<boost::uuids::uuid>(),
+        .updateTime = updateTime.value(),
+        .dbRole = static_cast<ClioNode::DbRole>(dbRoleValue)
+    };
 }
 
 }  // namespace cluster

src/cluster/ClioNode.hpp

@@ -19,6 +19,8 @@
 
 #pragma once
 
+#include "etl/WriterState.hpp"
+
 #include <boost/json/conversion.hpp>
 #include <boost/json/value.hpp>
 #include <boost/uuid/uuid.hpp>
@@ -37,16 +39,37 @@ struct ClioNode {
      */
     static constexpr char const* kTIME_FORMAT = "%Y-%m-%dT%H:%M:%SZ";
 
-    // enum class WriterRole {
-    //     ReadOnly,
-    //     NotWriter,
-    //     Writer
-    // };
+    /**
+     * @brief Database role of a node in the cluster.
+     *
+     * Roles are used to coordinate which node writes to the database:
+     * - ReadOnly: Node is configured to never write (strict read-only mode)
+     * - NotWriter: Node can write but is currently not the designated writer
+     * - Writer: Node is actively writing to the database
+     * - Fallback: Node is using the fallback writer decision mechanism
+     *
+     * When any node in the cluster is in Fallback mode, the entire cluster switches
+     * from the cluster communication mechanism to the slower but more reliable
+     * database-based conflict detection mechanism.
+     */
+    enum class DbRole { ReadOnly = 0, LoadingCache = 1, NotWriter = 2, Writer = 3, Fallback = 4, MAX = 4 };
 
-    std::shared_ptr<boost::uuids::uuid> uuid;          ///< The UUID of the node.
+    using Uuid = std::shared_ptr<boost::uuids::uuid>;
+    using CUuid = std::shared_ptr<boost::uuids::uuid const>;
+
+    Uuid uuid;                                         ///< The UUID of the node.
     std::chrono::system_clock::time_point updateTime;  ///< The time the data about the node was last updated.
+    DbRole dbRole;                                     ///< The database role of the node
 
-    // WriterRole writerRole;
+    /**
+     * @brief Create a ClioNode from writer state.
+     *
+     * @param uuid The UUID of the node
+     * @param writerState The writer state to determine the node's database role
+     * @return A ClioNode with the current time and role derived from writerState
+     */
+    static ClioNode
+    from(Uuid uuid, etl::WriterStateInterface const& writerState);
 };
 
 void

src/cluster/ClusterCommunicationService.cpp

@@ -19,98 +19,37 @@
 
 #include "cluster/ClusterCommunicationService.hpp"
 
-#include "cluster/ClioNode.hpp"
 #include "data/BackendInterface.hpp"
-#include "util/Assert.hpp"
-#include "util/Spawn.hpp"
-#include "util/log/Logger.hpp"
-
-#include <boost/asio/bind_cancellation_slot.hpp>
-#include <boost/asio/cancellation_type.hpp>
-#include <boost/asio/error.hpp>
-#include <boost/asio/spawn.hpp>
-#include <boost/asio/steady_timer.hpp>
-#include <boost/asio/use_future.hpp>
-#include <boost/json/parse.hpp>
-#include <boost/json/serialize.hpp>
-#include <boost/json/value.hpp>
-#include <boost/json/value_from.hpp>
-#include <boost/json/value_to.hpp>
-#include <boost/uuid/random_generator.hpp>
-#include <boost/uuid/uuid.hpp>
+#include "etl/WriterState.hpp"
 
 #include <chrono>
 #include <ctime>
-#include <latch>
 #include <memory>
-#include <string>
 #include <utility>
-#include <vector>
-
-namespace {
-constexpr auto kTOTAL_WORKERS = 2uz;  // 1 reading and 1 writing worker (coroutines)
-}  // namespace
 
 namespace cluster {
 
 ClusterCommunicationService::ClusterCommunicationService(
     std::shared_ptr<data::BackendInterface> backend,
+    std::unique_ptr<etl::WriterStateInterface> writerState,
     std::chrono::steady_clock::duration readInterval,
     std::chrono::steady_clock::duration writeInterval
 )
-    : backend_(std::move(backend))
-    , readInterval_(readInterval)
-    , writeInterval_(writeInterval)
-    , finishedCountdown_(kTOTAL_WORKERS)
-    , selfData_{ClioNode{
-          .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator{}()),
-          .updateTime = std::chrono::system_clock::time_point{}
-      }}
+    : backend_(ctx_, std::move(backend), writerState->clone(), readInterval, writeInterval)
+    , writerDecider_(ctx_, std::move(writerState))
 {
-    nodesInClusterMetric_.set(1);  // The node always sees itself
-    isHealthy_ = true;
 }
 
 void
 ClusterCommunicationService::run()
 {
-    ASSERT(not running_ and not stopped_, "Can only be ran once");
-    running_ = true;
-
-    util::spawn(strand_, [this](boost::asio::yield_context yield) {
-        boost::asio::steady_timer timer(yield.get_executor());
-        boost::system::error_code ec;
-
-        while (running_) {
-            timer.expires_after(readInterval_);
-            auto token = cancelSignal_.slot();
-            timer.async_wait(boost::asio::bind_cancellation_slot(token, yield[ec]));
-
-            if (ec == boost::asio::error::operation_aborted or not running_)
-                break;
-
-            doRead(yield);
-        }
-
-        finishedCountdown_.count_down(1);
+    backend_.subscribeToNewState([this](auto&&... args) {
+        metrics_.onNewState(std::forward<decltype(args)>(args)...);
     });
-
-    util::spawn(strand_, [this](boost::asio::yield_context yield) {
-        boost::asio::steady_timer timer(yield.get_executor());
-        boost::system::error_code ec;
-
-        while (running_) {
-            doWrite();
-            timer.expires_after(writeInterval_);
-            auto token = cancelSignal_.slot();
-            timer.async_wait(boost::asio::bind_cancellation_slot(token, yield[ec]));
-
-            if (ec == boost::asio::error::operation_aborted or not running_)
-                break;
-        }
-
-        finishedCountdown_.count_down(1);
+    backend_.subscribeToNewState([this](auto&&... args) {
+        writerDecider_.onNewState(std::forward<decltype(args)>(args)...);
     });
+    backend_.run();
 }
 
 ClusterCommunicationService::~ClusterCommunicationService()
@@ -121,107 +60,7 @@ ClusterCommunicationService::~ClusterCommunicationService()
 void
 ClusterCommunicationService::stop()
 {
-    if (stopped_)
-        return;
-
-    stopped_ = true;
-
-    // for ASAN to see through concurrency correctly we need to exit all coroutines before joining the ctx
-    running_ = false;
-
-    // cancelSignal_ is not thread safe so we execute emit on the same strand
-    boost::asio::spawn(
-        strand_, [this](auto&&) { cancelSignal_.emit(boost::asio::cancellation_type::all); }, boost::asio::use_future
-    )
-        .wait();
-    finishedCountdown_.wait();
-
-    ctx_.join();
-}
-
-std::shared_ptr<boost::uuids::uuid>
-ClusterCommunicationService::selfUuid() const
-{
-    // Uuid never changes so it is safe to copy it without using strand_
-    return selfData_.uuid;
-}
-
-ClioNode
-ClusterCommunicationService::selfData() const
-{
-    ClioNode result{};
-    util::spawn(strand_, [this, &result](boost::asio::yield_context) { result = selfData_; });
-    return result;
-}
-
-std::expected<std::vector<ClioNode>, std::string>
-ClusterCommunicationService::clusterData() const
-{
-    if (not isHealthy_) {
-        return std::unexpected{"Service is not healthy"};
-    }
-    std::vector<ClioNode> result;
-    util::spawn(strand_, [this, &result](boost::asio::yield_context) {
-        result = otherNodesData_;
-        result.push_back(selfData_);
-    });
-    return result;
-}
-
-void
-ClusterCommunicationService::doRead(boost::asio::yield_context yield)
-{
-    otherNodesData_.clear();
-
-    BackendInterface::ClioNodesDataFetchResult expectedResult;
-    try {
-        expectedResult = backend_->fetchClioNodesData(yield);
-    } catch (...) {
-        expectedResult = std::unexpected{"Failed to fecth Clio nodes data"};
-    }
-
-    if (!expectedResult.has_value()) {
-        LOG(log_.error()) << "Failed to fetch nodes data";
-        isHealthy_ = false;
-        return;
-    }
-
-    // Create a new vector here to not have partially parsed data in otherNodesData_
-    std::vector<ClioNode> otherNodesData;
-    for (auto const& [uuid, nodeDataStr] : expectedResult.value()) {
-        if (uuid == *selfData_.uuid) {
-            continue;
-        }
-
-        boost::system::error_code errorCode;
-        auto const json = boost::json::parse(nodeDataStr, errorCode);
-        if (errorCode.failed()) {
-            LOG(log_.error()) << "Error parsing json from DB: " << nodeDataStr;
-            isHealthy_ = false;
-            return;
-        }
-
-        auto expectedNodeData = boost::json::try_value_to<ClioNode>(json);
-        if (expectedNodeData.has_error()) {
-            LOG(log_.error()) << "Error converting json to ClioNode: " << json;
-            isHealthy_ = false;
-            return;
-        }
-        *expectedNodeData->uuid = uuid;
-        otherNodesData.push_back(std::move(expectedNodeData).value());
-    }
-    otherNodesData_ = std::move(otherNodesData);
-    nodesInClusterMetric_.set(otherNodesData_.size() + 1);
-    isHealthy_ = true;
-}
-
-void
-ClusterCommunicationService::doWrite()
-{
-    selfData_.updateTime = std::chrono::system_clock::now();
-    boost::json::value jsonValue{};
-    boost::json::value_from(selfData_, jsonValue);
-    backend_->writeNodeMessage(*selfData_.uuid, boost::json::serialize(jsonValue.as_object()));
+    backend_.stop();
 }
 
 }  // namespace cluster

src/cluster/ClusterCommunicationService.hpp

@@ -19,13 +19,12 @@
 
 #pragma once
 
-#include "cluster/ClioNode.hpp"
-#include "cluster/ClusterCommunicationServiceInterface.hpp"
+#include "cluster/Backend.hpp"
+#include "cluster/Concepts.hpp"
+#include "cluster/Metrics.hpp"
+#include "cluster/WriterDecider.hpp"
 #include "data/BackendInterface.hpp"
-#include "util/log/Logger.hpp"
-#include "util/prometheus/Bool.hpp"
-#include "util/prometheus/Gauge.hpp"
-#include "util/prometheus/Prometheus.hpp"
+#include "etl/WriterState.hpp"
 
 #include <boost/asio/cancellation_signal.hpp>
 #include <boost/asio/spawn.hpp>
@@ -33,67 +32,49 @@
 #include <boost/asio/thread_pool.hpp>
 #include <boost/uuid/uuid.hpp>
 
-#include <atomic>
 #include <chrono>
-#include <latch>
 #include <memory>
-#include <string>
-#include <vector>
 
 namespace cluster {
 
 /**
  * @brief Service to post and read messages to/from the cluster. It uses a backend to communicate with the cluster.
  */
-class ClusterCommunicationService : public ClusterCommunicationServiceInterface {
-    util::prometheus::GaugeInt& nodesInClusterMetric_ = PrometheusService::gaugeInt(
-        "cluster_nodes_total_number",
-        {},
-        "Total number of nodes this node can detect in the cluster."
-    );
-    util::prometheus::Bool isHealthy_ = PrometheusService::boolMetric(
-        "cluster_communication_is_healthy",
-        {},
-        "Whether cluster communication service is operating healthy (1 - healthy, 0 - we have a problem)"
-    );
-
+class ClusterCommunicationService : public ClusterCommunicationServiceTag {
     // TODO: Use util::async::CoroExecutionContext after https://github.com/XRPLF/clio/issues/1973 is implemented
     boost::asio::thread_pool ctx_{1};
-    boost::asio::strand<boost::asio::thread_pool::executor_type> strand_ = boost::asio::make_strand(ctx_);
-
-    util::Logger log_{"ClusterCommunication"};
-
-    std::shared_ptr<data::BackendInterface> backend_;
-
-    std::chrono::steady_clock::duration readInterval_;
-    std::chrono::steady_clock::duration writeInterval_;
-
-    boost::asio::cancellation_signal cancelSignal_;
-    std::latch finishedCountdown_;
-    std::atomic_bool running_ = false;
-    bool stopped_ = false;
-
-    ClioNode selfData_;
-    std::vector<ClioNode> otherNodesData_;
+    Backend backend_;
+    Metrics metrics_;
+    WriterDecider writerDecider_;
 
 public:
-    static constexpr std::chrono::milliseconds kDEFAULT_READ_INTERVAL{2100};
-    static constexpr std::chrono::milliseconds kDEFAULT_WRITE_INTERVAL{1200};
+    static constexpr std::chrono::milliseconds kDEFAULT_READ_INTERVAL{1000};
+    static constexpr std::chrono::milliseconds kDEFAULT_WRITE_INTERVAL{1000};
+
     /**
      * @brief Construct a new Cluster Communication Service object.
      *
      * @param backend The backend to use for communication.
+     * @param writerState The state showing whether clio is writing to the database.
      * @param readInterval The interval to read messages from the cluster.
      * @param writeInterval The interval to write messages to the cluster.
      */
     ClusterCommunicationService(
         std::shared_ptr<data::BackendInterface> backend,
+        std::unique_ptr<etl::WriterStateInterface> writerState,
         std::chrono::steady_clock::duration readInterval = kDEFAULT_READ_INTERVAL,
         std::chrono::steady_clock::duration writeInterval = kDEFAULT_WRITE_INTERVAL
     );
 
     ~ClusterCommunicationService() override;
 
+    ClusterCommunicationService(ClusterCommunicationService&&) = delete;
+    ClusterCommunicationService(ClusterCommunicationService const&) = delete;
+    ClusterCommunicationService&
+    operator=(ClusterCommunicationService&&) = delete;
+    ClusterCommunicationService&
+    operator=(ClusterCommunicationService const&) = delete;
+
     /**
      * @brief Start the service.
      */
@@ -105,44 +86,6 @@ public:
      */
     void
     stop();
-
-    ClusterCommunicationService(ClusterCommunicationService&&) = delete;
-    ClusterCommunicationService(ClusterCommunicationService const&) = delete;
-    ClusterCommunicationService&
-    operator=(ClusterCommunicationService&&) = delete;
-    ClusterCommunicationService&
-    operator=(ClusterCommunicationService const&) = delete;
-
-    /**
-     * @brief Get the UUID of the current node.
-     *
-     * @return The UUID of the current node.
-     */
-    std::shared_ptr<boost::uuids::uuid>
-    selfUuid() const;
-
-    /**
-     * @brief Get the data of the current node.
-     *
-     * @return The data of the current node.
-     */
-    ClioNode
-    selfData() const override;
-
-    /**
-     * @brief Get the data of all nodes in the cluster (including self).
-     *
-     * @return The data of all nodes in the cluster or error if the service is not healthy.
-     */
-    std::expected<std::vector<ClioNode>, std::string>
-    clusterData() const override;
-
-private:
-    void
-    doRead(boost::asio::yield_context yield);
-
-    void
-    doWrite();
 };
 
 }  // namespace cluster

src/cluster/Concepts.hpp

@@ -0,0 +1,39 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include <concepts>
+
+namespace cluster {
+
+/**
+ * @brief Tag type for cluster communication service implementations.
+ *
+ * This tag is used to identify types that implement cluster communication functionality.
+ * Types should inherit from this tag to be recognized as cluster communication services.
+ */
+struct ClusterCommunicationServiceTag {
+    virtual ~ClusterCommunicationServiceTag() = default;
+};
+
+template <typename T>
+concept SomeClusterCommunicationService = std::derived_from<T, ClusterCommunicationServiceTag>;
+
+}  // namespace cluster

src/cluster/Metrics.cpp

@@ -17,38 +17,31 @@
 */
 //==============================================================================
 
-#pragma once
+#include "cluster/Metrics.hpp"
 
+#include "cluster/Backend.hpp"
 #include "cluster/ClioNode.hpp"
 
-#include <expected>
-#include <string>
-#include <vector>
+#include <memory>
 
 namespace cluster {
 
-/**
- * @brief Interface for the cluster communication service.
- */
-class ClusterCommunicationServiceInterface {
-public:
-    virtual ~ClusterCommunicationServiceInterface() = default;
+Metrics::Metrics()
+{
+    nodesInClusterMetric_.set(1);  // The node always sees itself
+    isHealthy_ = true;
+}
 
-    /**
-     * @brief Get the data of the current node.
-     *
-     * @return The data of the current node.
-     */
-    [[nodiscard]] virtual ClioNode
-    selfData() const = 0;
-
-    /**
-     * @brief Get the data of all nodes in the cluster (including self).
-     *
-     * @return The data of all nodes in the cluster or error if the service is not healthy.
-     */
-    [[nodiscard]] virtual std::expected<std::vector<ClioNode>, std::string>
-    clusterData() const = 0;
-};
+void
+Metrics::onNewState(ClioNode::CUuid, std::shared_ptr<Backend::ClusterData const> clusterData)
+{
+    if (clusterData->has_value()) {
+        isHealthy_ = true;
+        nodesInClusterMetric_.set(clusterData->value().size());
+    } else {
+        isHealthy_ = false;
+        nodesInClusterMetric_.set(1);
+    }
+}
 
 }  // namespace cluster

src/cluster/Metrics.hpp

@@ -0,0 +1,76 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "util/prometheus/Bool.hpp"
+#include "util/prometheus/Gauge.hpp"
+#include "util/prometheus/Prometheus.hpp"
+
+#include <memory>
+
+namespace cluster {
+
+/**
+ * @brief Manages Prometheus metrics for cluster communication and node tracking.
+ *
+ * This class tracks cluster-related metrics including:
+ * - Total number of nodes detected in the cluster
+ * - Health status of cluster communication
+ */
+class Metrics {
+    /** @brief Gauge tracking the total number of nodes visible in the cluster */
+    util::prometheus::GaugeInt& nodesInClusterMetric_ = PrometheusService::gaugeInt(
+        "cluster_nodes_total_number",
+        {},
+        "Total number of nodes this node can detect in the cluster."
+    );
+
+    /** @brief Boolean metric indicating whether cluster communication is healthy */
+    util::prometheus::Bool isHealthy_ = PrometheusService::boolMetric(
+        "cluster_communication_is_healthy",
+        {},
+        "Whether cluster communication service is operating healthy (1 - healthy, 0 - we have a problem)"
+    );
+
+public:
+    /**
+     * @brief Constructs a Metrics instance and initializes metrics.
+     *
+     * Sets the initial node count to 1 (self) and marks communication as healthy.
+     */
+    Metrics();
+
+    /**
+     * @brief Updates metrics based on new cluster state.
+     *
+     * This callback is invoked when cluster state changes. It updates:
+     * - Health status based on whether cluster data is available
+     * - Node count to reflect the current cluster size
+     *
+     * @param uuid The UUID of the node (unused in current implementation)
+     * @param clusterData Shared pointer to the current cluster data; may be empty if communication failed
+     */
+    void
+    onNewState(ClioNode::CUuid uuid, std::shared_ptr<Backend::ClusterData const> clusterData);
+};
+
+}  // namespace cluster

src/cluster/WriterDecider.cpp

@@ -0,0 +1,98 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/WriterDecider.hpp"
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "etl/WriterState.hpp"
+#include "util/Assert.hpp"
+#include "util/Spawn.hpp"
+
+#include <boost/asio/thread_pool.hpp>
+
+#include <algorithm>
+#include <memory>
+#include <utility>
+#include <vector>
+
+namespace cluster {
+
+WriterDecider::WriterDecider(boost::asio::thread_pool& ctx, std::unique_ptr<etl::WriterStateInterface> writerState)
+    : ctx_(ctx), writerState_(std::move(writerState))
+{
+}
+
+void
+WriterDecider::onNewState(ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData)
+{
+    if (not clusterData->has_value())
+        return;
+
+    util::spawn(
+        ctx_,
+        [writerState = writerState_->clone(),
+         selfId = std::move(selfId),
+         clusterData = clusterData->value()](auto&&) mutable {
+            auto const selfData =
+                std::ranges::find_if(clusterData, [&selfId](ClioNode const& node) { return node.uuid == selfId; });
+            ASSERT(selfData != clusterData.end(), "Self data should always be in the cluster data");
+
+            if (selfData->dbRole == ClioNode::DbRole::Fallback) {
+                return;
+            }
+
+            if (selfData->dbRole == ClioNode::DbRole::ReadOnly) {
+                writerState->giveUpWriting();
+                return;
+            }
+
+            // If any node in the cluster is in Fallback mode, the entire cluster must switch
+            // to the fallback writer decision mechanism for consistency
+            if (std::ranges::any_of(clusterData, [](ClioNode const& node) {
+                    return node.dbRole == ClioNode::DbRole::Fallback;
+                })) {
+                writerState->setWriterDecidingFallback();
+                return;
+            }
+
+            // We are not ReadOnly and there is no Fallback in the cluster
+            std::ranges::sort(clusterData, [](ClioNode const& lhs, ClioNode const& rhs) {
+                return *lhs.uuid < *rhs.uuid;
+            });
+
+            auto const it = std::ranges::find_if(clusterData, [](ClioNode const& node) {
+                return node.dbRole == ClioNode::DbRole::NotWriter or node.dbRole == ClioNode::DbRole::Writer;
+            });
+
+            if (it == clusterData.end()) {
+                // No writer nodes in the cluster yet
+                return;
+            }
+
+            if (*it->uuid == *selfId) {
+                writerState->startWriting();
+            } else {
+                writerState->giveUpWriting();
+            }
+        }
+    );
+}
+
+}  // namespace cluster

src/cluster/WriterDecider.hpp

@@ -0,0 +1,75 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "etl/WriterState.hpp"
+
+#include <boost/asio/thread_pool.hpp>
+
+#include <memory>
+
+namespace cluster {
+
+/**
+ * @brief Decides which node in the cluster should be the writer based on cluster state.
+ *
+ * This class monitors cluster state changes and determines whether the current node
+ * should act as the writer to the database. The decision is made by:
+ * 1. Sorting all nodes by UUID for deterministic ordering
+ * 2. Selecting the first node that is allowed to write (not ReadOnly)
+ * 3. Activating writing on this node if it's the current node, otherwise deactivating
+ *
+ * This ensures only one node in the cluster actively writes to the database at a time.
+ */
+class WriterDecider {
+    /** @brief Thread pool for spawning asynchronous tasks */
+    boost::asio::thread_pool& ctx_;
+
+    /** @brief Interface for controlling the writer state of this node */
+    std::unique_ptr<etl::WriterStateInterface> writerState_;
+
+public:
+    /**
+     * @brief Constructs a WriterDecider.
+     *
+     * @param ctx Thread pool for executing asynchronous operations
+     * @param writerState Writer state interface for controlling write operations
+     */
+    WriterDecider(boost::asio::thread_pool& ctx, std::unique_ptr<etl::WriterStateInterface> writerState);
+
+    /**
+     * @brief Handles cluster state changes and decides whether this node should be the writer.
+     *
+     * This method is called when cluster state changes. It asynchronously:
+     * - Sorts all nodes by UUID to establish a deterministic order
+     * - Identifies the first node allowed to write (not ReadOnly)
+     * - Activates writing if this node is selected, otherwise deactivates writing
+     * - Logs a warning if no nodes in the cluster are allowed to write
+     *
+     * @param selfId The UUID of the current node
+     * @param clusterData Shared pointer to current cluster data; may be empty if communication failed
+     */
+    void
+    onNewState(ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData);
+};
+
+}  // namespace cluster

src/cluster/impl/RepeatedTask.hpp

@@ -0,0 +1,104 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "util/Assert.hpp"
+#include "util/Spawn.hpp"
+
+#include <boost/asio/bind_cancellation_slot.hpp>
+#include <boost/asio/cancellation_signal.hpp>
+#include <boost/asio/cancellation_type.hpp>
+#include <boost/asio/error.hpp>
+#include <boost/asio/executor.hpp>
+#include <boost/asio/spawn.hpp>
+#include <boost/asio/steady_timer.hpp>
+#include <boost/asio/strand.hpp>
+#include <boost/asio/use_future.hpp>
+
+#include <atomic>
+#include <chrono>
+#include <concepts>
+#include <semaphore>
+
+namespace cluster::impl {
+
+// TODO: Try to replace util::Repeat by this. https://github.com/XRPLF/clio/issues/2926
+template <typename Context>
+class RepeatedTask {
+    std::chrono::steady_clock::duration interval_;
+    boost::asio::strand<typename Context::executor_type> strand_;
+
+    enum class State { Running, Stopped };
+    std::atomic<State> state_ = State::Stopped;
+
+    std::binary_semaphore semaphore_{0};
+    boost::asio::steady_timer timer_;
+
+public:
+    RepeatedTask(std::chrono::steady_clock::duration interval, Context& ctx)
+        : interval_(interval), strand_(boost::asio::make_strand(ctx)), timer_(strand_)
+    {
+    }
+
+    ~RepeatedTask()
+    {
+        stop();
+    }
+
+    template <typename Fn>
+        requires std::invocable<Fn, boost::asio::yield_context> or std::invocable<Fn>
+    void
+    run(Fn&& f)
+    {
+        ASSERT(state_ == State::Stopped, "Can only be ran once");
+        state_ = State::Running;
+        util::spawn(strand_, [this, f = std::forward<Fn>(f)](boost::asio::yield_context yield) {
+            boost::system::error_code ec;
+
+            while (state_ == State::Running) {
+                timer_.expires_after(interval_);
+                timer_.async_wait(yield[ec]);
+
+                if (ec or state_ != State::Running)
+                    break;
+
+                if constexpr (std::invocable<decltype(f), boost::asio::yield_context>) {
+                    f(yield);
+                } else {
+                    f();
+                }
+            }
+
+            semaphore_.release();
+        });
+    }
+
+    void
+    stop()
+    {
+        if (auto expected = State::Running; not state_.compare_exchange_strong(expected, State::Stopped))
+            return;  // Already stopped or not started
+
+        boost::asio::spawn(strand_, [this](auto&&) { timer_.cancel(); }, boost::asio::use_future).wait();
+        semaphore_.acquire();
+    }
+};
+
+}  // namespace cluster::impl

src/data/impl/LedgerCacheFile.cpp

@@ -115,6 +115,11 @@ LedgerCacheFile::write(DataView dataView)
     auto const hash = file.hash();
     file.write(hash.data(), decltype(hash)::bytes);
 
+    // flush internal buffer explicitly before renaming
+    if (auto const expectedSuccess = file.close(); not expectedSuccess.has_value()) {
+        return expectedSuccess;
+    }
+
     try {
         std::filesystem::rename(newFilePath, path_);
     } catch (std::exception const& e) {

src/data/impl/OutputFile.cpp

@@ -23,6 +23,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <expected>
 #include <ios>
 #include <string>
 #include <utility>
@@ -59,4 +60,14 @@ OutputFile::hash() const
     return std::move(sum).finalize();
 }
 
+std::expected<void, std::string>
+OutputFile::close()
+{
+    file_.close();
+    if (not file_) {
+        return std::unexpected{"Error closing cache file"};
+    }
+    return {};
+}
+
 }  // namespace data::impl

src/data/impl/OutputFile.hpp

@@ -25,6 +25,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <expected>
 #include <fstream>
 #include <string>
 
@@ -60,6 +61,9 @@ public:
     ripple::uint256
     hash() const;
 
+    std::expected<void, std::string>
+    close();
+
 private:
     void
     writeToFile(char const* data, size_t size);

src/etl/CMakeLists.txt

@@ -11,6 +11,7 @@ target_sources(
           NetworkValidatedLedgers.cpp
           NFTHelpers.cpp
           Source.cpp
+          WriterState.cpp
           impl/AmendmentBlockHandler.cpp
           impl/AsyncGrpcCall.cpp
           impl/Extraction.cpp

src/etl/ETLService.cpp

@@ -78,6 +78,7 @@ namespace etl {
 std::shared_ptr<ETLServiceInterface>
 ETLService::makeETLService(
     util::config::ClioConfigDefinition const& config,
+    std::shared_ptr<SystemState> state,
     util::async::AnyExecutionContext ctx,
     std::shared_ptr<BackendInterface> backend,
     std::shared_ptr<feed::SubscriptionManagerInterface> subscriptions,
@@ -87,9 +88,6 @@ ETLService::makeETLService(
 {
     std::shared_ptr<ETLServiceInterface> ret;
 
-    auto state = std::make_shared<SystemState>();
-    state->isStrictReadonly = config.get<bool>("read_only");
-
     auto fetcher = std::make_shared<impl::LedgerFetcher>(backend, balancer);
     auto extractor = std::make_shared<impl::Extractor>(fetcher);
     auto publisher = std::make_shared<impl::LedgerPublisher>(ctx, backend, subscriptions, *state);
@@ -173,6 +171,7 @@ ETLService::ETLService(
     , state_(std::move(state))
     , startSequence_(config.get().maybeValue<uint32_t>("start_sequence"))
     , finishSequence_(config.get().maybeValue<uint32_t>("finish_sequence"))
+    , writeCommandStrand_(ctx_.makeStrand())
 {
     ASSERT(not state_->isWriting, "ETL should never start in writer mode");
 
@@ -213,14 +212,13 @@ ETLService::run()
             return;
         }
 
-        auto nextSequence = rng->maxSequence + 1;
-        if (backend_->cache().latestLedgerSequence() != 0) {
-            nextSequence = backend_->cache().latestLedgerSequence();
-        }
-
+        auto const nextSequence = syncCacheWithDb();
         LOG(log_.debug()) << "Database is populated. Starting monitor loop. sequence = " << nextSequence;
+
         startMonitor(nextSequence);
 
+        state_->isLoadingCache = false;
+
         // If we are a writer as the result of loading the initial ledger - start loading
         if (state_->isWriting)
             startLoading(nextSequence);
@@ -232,6 +230,13 @@ ETLService::stop()
 {
     LOG(log_.info()) << "Stop called";
 
+    systemStateWriteCommandSubscription_.disconnect();
+    auto count = runningWriteCommandHandlers_.load();
+    while (count != 0) {
+        runningWriteCommandHandlers_.wait(count);  // Blocks until value changes
+        count = runningWriteCommandHandlers_.load();
+    }
+
     if (mainLoop_)
         mainLoop_->wait();
     if (taskMan_)
@@ -343,35 +348,77 @@ ETLService::loadInitialLedgerIfNeeded()
     return rng;
 }
 
+uint32_t
+ETLService::syncCacheWithDb()
+{
+    auto rng = backend_->hardFetchLedgerRangeNoThrow();
+
+    while (not backend_->cache().isDisabled() and rng->maxSequence > backend_->cache().latestLedgerSequence()) {
+        LOG(log_.info()) << "Syncing cache with DB. DB latest seq: " << rng->maxSequence
+                         << ". Cache latest seq: " << backend_->cache().latestLedgerSequence();
+        for (auto seq = backend_->cache().latestLedgerSequence(); seq <= rng->maxSequence; ++seq) {
+            LOG(log_.info()) << "ETLService (via syncCacheWithDb) got new seq from db: " << seq;
+            updateCache(seq);
+        }
+        rng = backend_->hardFetchLedgerRangeNoThrow();
+    }
+    return rng->maxSequence + 1;
+}
+
+void
+ETLService::updateCache(uint32_t seq)
+{
+    auto const cacheNeedsUpdate = backend_->cache().latestLedgerSequence() < seq;
+    auto const backendRange = backend_->fetchLedgerRange();
+    auto const backendNeedsUpdate = backendRange.has_value() and backendRange->maxSequence < seq;
+
+    if (cacheNeedsUpdate) {
+        auto const diff = data::synchronousAndRetryOnTimeout([this, seq](auto yield) {
+            return backend_->fetchLedgerDiff(seq, yield);
+        });
+        cacheUpdater_->update(seq, diff);
+    }
+
+    if (backendNeedsUpdate)
+        backend_->updateRange(seq);
+
+    publisher_->publish(seq, {});
+}
+
 void
 ETLService::startMonitor(uint32_t seq)
 {
     monitor_ = monitorProvider_->make(ctx_, backend_, ledgers_, seq);
 
+    systemStateWriteCommandSubscription_ =
+        state_->writeCommandSignal.connect([this](SystemState::WriteCommand command) {
+            ++runningWriteCommandHandlers_;
+            writeCommandStrand_.submit([this, command]() {
+                switch (command) {
+                    case etl::SystemState::WriteCommand::StartWriting:
+                        attemptTakeoverWriter();
+                        break;
+                    case etl::SystemState::WriteCommand::StopWriting:
+                        giveUpWriter();
+                        break;
+                }
+                --runningWriteCommandHandlers_;
+                runningWriteCommandHandlers_.notify_one();
+            });
+        });
+
     monitorNewSeqSubscription_ = monitor_->subscribeToNewSequence([this](uint32_t seq) {
         LOG(log_.info()) << "ETLService (via Monitor) got new seq from db: " << seq;
-
-        if (state_->writeConflict) {
-            LOG(log_.info()) << "Got a write conflict; Giving up writer seat immediately";
-            giveUpWriter();
-        }
-
-        if (not state_->isWriting) {
-            auto const diff = data::synchronousAndRetryOnTimeout([this, seq](auto yield) {
-                return backend_->fetchLedgerDiff(seq, yield);
-            });
-
-            cacheUpdater_->update(seq, diff);
-            backend_->updateRange(seq);
-        }
-
-        publisher_->publish(seq, {});
+        updateCache(seq);
     });
 
     monitorDbStalledSubscription_ = monitor_->subscribeToDbStalled([this]() {
         LOG(log_.warn()) << "ETLService received DbStalled signal from Monitor";
+        // Database stall detected - no writer has been active for 10 seconds
+        // This triggers the fallback mechanism and attempts to become the writer
         if (not state_->isStrictReadonly and not state_->isWriting)
-            attemptTakeoverWriter();
+            state_->writeCommandSignal(SystemState::WriteCommand::StartWriting);
+        state_->isWriterDecidingFallback = true;
     });
 
     monitor_->run();
@@ -394,6 +441,13 @@ ETLService::attemptTakeoverWriter()
     auto rng = backend_->hardFetchLedgerRangeNoThrow();
     ASSERT(rng.has_value(), "Ledger range can't be null");
 
+    if (backend_->cache().latestLedgerSequence() != rng->maxSequence) {
+        LOG(log_.info()) << "Wanted to take over the ETL writer seat but LedgerCache is outdated";
+        // Give ETL time to update LedgerCache. This method will be called because ClusterCommunication will likely to
+        // continue sending StartWriting signal every 1 second
+        return;
+    }
+
     state_->isWriting = true;  // switch to writer
     LOG(log_.info()) << "Taking over the ETL writer seat";
     startLoading(rng->maxSequence + 1);
@@ -404,7 +458,7 @@ ETLService::giveUpWriter()
 {
     ASSERT(not state_->isStrictReadonly, "This should only happen on writer nodes");
     state_->isWriting = false;
-    state_->writeConflict = false;
+    LOG(log_.info()) << "Giving up writer seat";
     taskMan_ = nullptr;
 }
 

src/etl/ETLService.hpp

@@ -52,6 +52,7 @@
 #include "feed/SubscriptionManagerInterface.hpp"
 #include "util/async/AnyExecutionContext.hpp"
 #include "util/async/AnyOperation.hpp"
+#include "util/async/AnyStrand.hpp"
 #include "util/config/ConfigDefinition.hpp"
 #include "util/log/Logger.hpp"
 
@@ -69,12 +70,12 @@
 #include <xrpl/protocol/TxFormats.h>
 #include <xrpl/protocol/TxMeta.h>
 
+#include <atomic>
 #include <cstddef>
 #include <cstdint>
 #include <functional>
 #include <memory>
 #include <optional>
-#include <string>
 
 namespace etl {
 
@@ -117,6 +118,9 @@ class ETLService : public ETLServiceInterface {
 
     boost::signals2::scoped_connection monitorNewSeqSubscription_;
     boost::signals2::scoped_connection monitorDbStalledSubscription_;
+    boost::signals2::scoped_connection systemStateWriteCommandSubscription_;
+    util::async::AnyStrand writeCommandStrand_;
+    std::atomic<size_t> runningWriteCommandHandlers_{0};
 
     std::optional<util::async::AnyOperation<void>> mainLoop_;
 
@@ -127,6 +131,7 @@ public:
      * Creates and runs the ETL service.
      *
      * @param config The configuration to use
+     * @param state The system state tracking object
      * @param ctx Execution context for asynchronous operations
      * @param backend BackendInterface implementation
      * @param subscriptions Subscription manager
@@ -137,6 +142,7 @@ public:
     static std::shared_ptr<ETLServiceInterface>
     makeETLService(
         util::config::ClioConfigDefinition const& config,
+        std::shared_ptr<SystemState> state,
         util::async::AnyExecutionContext ctx,
         std::shared_ptr<BackendInterface> backend,
         std::shared_ptr<feed::SubscriptionManagerInterface> subscriptions,
@@ -160,7 +166,7 @@ public:
      * @param initialLoadObserver The observer for initial data loading
      * @param taskManagerProvider The provider of the task manager instance
      * @param monitorProvider The provider of the monitor instance
-     * @param state System state tracking object
+     * @param state The system state tracking object
      */
     ETLService(
         util::async::AnyExecutionContext ctx,
@@ -206,6 +212,12 @@ private:
     std::optional<data::LedgerRange>
     loadInitialLedgerIfNeeded();
 
+    [[nodiscard]] uint32_t
+    syncCacheWithDb();
+
+    void
+    updateCache(uint32_t seq);
+
     void
     startMonitor(uint32_t seq);
 

src/etl/SystemState.hpp

@@ -19,11 +19,16 @@
 
 #pragma once
 
+#include "util/config/ConfigDefinition.hpp"
+#include "util/log/Logger.hpp"
 #include "util/prometheus/Bool.hpp"
 #include "util/prometheus/Label.hpp"
 #include "util/prometheus/Prometheus.hpp"
 
-#include <atomic>
+#include <boost/signals2/signal.hpp>
+#include <boost/signals2/variadic_signal.hpp>
+
+#include <memory>
 
 namespace etl {
 
@@ -31,6 +36,25 @@ namespace etl {
  * @brief Represents the state of the ETL subsystem.
  */
 struct SystemState {
+    SystemState()
+    {
+        isLoadingCache = true;
+    }
+
+    /**
+     * @brief Factory method to create a SystemState instance.
+     *
+     * @param config The configuration to use for initializing the system state
+     * @return A shared pointer to the newly created SystemState
+     */
+    static std::shared_ptr<SystemState>
+    makeSystemState(util::config::ClioConfigDefinition const& config)
+    {
+        auto state = std::make_shared<SystemState>();
+        state->isStrictReadonly = config.get<bool>("read_only");
+        return state;
+    }
+
     /**
      * @brief Whether the process is in strict read-only mode.
      *
@@ -50,8 +74,31 @@ struct SystemState {
         "Whether the process is writing to the database"
     );
 
-    std::atomic_bool isStopping = false;    /**< @brief Whether the software is stopping. */
-    std::atomic_bool writeConflict = false; /**< @brief Whether a write conflict was detected. */
+    /** @brief Whether the process is still loading cache after startup. */
+    util::prometheus::Bool isLoadingCache = PrometheusService::boolMetric(
+        "etl_loading_cache",
+        util::prometheus::Labels{},
+        "Whether etl is loading cache after clio startup"
+    );
+
+    /**
+     * @brief Commands for controlling the ETL writer state.
+     *
+     * These commands are emitted via writeCommandSignal to coordinate writer state transitions across components.
+     */
+    enum class WriteCommand {
+        StartWriting, /**< Request to attempt taking over as the ETL writer */
+        StopWriting   /**< Request to give up the ETL writer role (e.g., due to write conflict) */
+    };
+
+    /**
+     * @brief Signal for coordinating ETL writer state transitions.
+     *
+     * This signal allows components to request changes to the writer state without direct coupling.
+     * - Emitted with StartWriting when database stalls and node should attempt to become writer
+     * - Emitted with StopWriting when write conflicts are detected
+     */
+    boost::signals2::signal<void(WriteCommand)> writeCommandSignal;
 
     /**
      * @brief Whether clio detected an amendment block.
@@ -77,6 +124,24 @@ struct SystemState {
         util::prometheus::Labels{},
         "Whether clio detected a corruption that needs manual attention"
     );
+
+    /**
+     * @brief Whether the cluster is using the fallback writer decision mechanism.
+     *
+     * The fallback mechanism is triggered when:
+     * - The database stalls for 10 seconds (detected by Monitor), indicating no active writer
+     * - A write conflict is detected, indicating multiple nodes attempting to write simultaneously
+     *
+     * When fallback mode is active, the cluster stops using the cluster communication mechanism
+     * (TTL-based role announcements) and relies on the slower but more reliable database-based
+     * conflict detection. This flag propagates across the cluster - if any node enters fallback
+     * mode, all nodes in the cluster will switch to fallback mode.
+     */
+    util::prometheus::Bool isWriterDecidingFallback = PrometheusService::boolMetric(
+        "etl_writing_deciding_fallback",
+        util::prometheus::Labels{},
+        "Whether the cluster is using the fallback writer decision mechanism"
+    );
 };
 
 }  // namespace etl

src/etl/WriterState.cpp

@@ -0,0 +1,88 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "etl/WriterState.hpp"
+
+#include "etl/SystemState.hpp"
+
+#include <memory>
+#include <utility>
+
+namespace etl {
+
+WriterState::WriterState(std::shared_ptr<SystemState> state) : systemState_(std::move(state))
+{
+}
+
+bool
+WriterState::isReadOnly() const
+{
+    return systemState_->isStrictReadonly;
+}
+
+bool
+WriterState::isWriting() const
+{
+    return systemState_->isWriting;
+}
+
+void
+WriterState::startWriting()
+{
+    if (isWriting())
+        return;
+
+    systemState_->writeCommandSignal(SystemState::WriteCommand::StartWriting);
+}
+
+void
+WriterState::giveUpWriting()
+{
+    if (not isWriting())
+        return;
+
+    systemState_->writeCommandSignal(SystemState::WriteCommand::StopWriting);
+}
+
+void
+WriterState::setWriterDecidingFallback()
+{
+    systemState_->isWriterDecidingFallback = true;
+}
+
+bool
+WriterState::isFallback() const
+{
+    return systemState_->isWriterDecidingFallback;
+}
+
+bool
+WriterState::isLoadingCache() const
+{
+    return systemState_->isLoadingCache;
+}
+
+std::unique_ptr<WriterStateInterface>
+WriterState::clone() const
+{
+    auto c = WriterState(*this);
+    return std::make_unique<WriterState>(std::move(c));
+}
+
+}  // namespace etl

src/etl/WriterState.hpp

@@ -0,0 +1,193 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "etl/SystemState.hpp"
+
+#include <memory>
+
+namespace etl {
+
+/**
+ * @brief Interface for managing writer state in the ETL subsystem.
+ *
+ * This interface provides methods to query and control whether the ETL process
+ * is actively writing to the database. Implementations should coordinate with
+ * the ETL system state to manage write responsibilities.
+ */
+class WriterStateInterface {
+public:
+    virtual ~WriterStateInterface() = default;
+
+    /**
+     * @brief Check if the ETL process is in strict read-only mode.
+     * @return true if the process is in strict read-only mode, false otherwise
+     */
+    [[nodiscard]] virtual bool
+    isReadOnly() const = 0;
+
+    /**
+     * @brief Check if the ETL process is currently writing to the database.
+     * @return true if the process is writing, false otherwise
+     */
+    [[nodiscard]] virtual bool
+    isWriting() const = 0;
+
+    /**
+     * @brief Request to start writing to the database.
+     *
+     * This method signals that the process should take over writing responsibilities.
+     * The actual transition to writing state may not be immediate.
+     */
+    virtual void
+    startWriting() = 0;
+
+    /**
+     * @brief Request to stop writing to the database.
+     *
+     * This method signals that the process should give up writing responsibilities.
+     * The actual transition from writing state may not be immediate.
+     */
+    virtual void
+    giveUpWriting() = 0;
+
+    /**
+     * @brief Check if the cluster is using the fallback writer decision mechanism.
+     *
+     * @return true if the cluster has switched to fallback mode, false otherwise
+     */
+    [[nodiscard]] virtual bool
+    isFallback() const = 0;
+
+    /**
+     * @brief Switch the cluster to the fallback writer decision mechanism.
+     *
+     * This method is called when the cluster needs to transition from the cluster
+     * communication mechanism to the slower but more reliable fallback mechanism.
+     * Once set, this flag propagates to all nodes in the cluster through the
+     * ClioNode DbRole::Fallback state.
+     */
+    virtual void
+    setWriterDecidingFallback() = 0;
+
+    /**
+     * @brief Whether clio is still loading cache after startup.
+     *
+     * @return true if clio is still loading cache, false otherwise.
+     */
+    [[nodiscard]] virtual bool
+    isLoadingCache() const = 0;
+
+    /**
+     * @brief Create a clone of this writer state.
+     *
+     * Creates a new instance of the writer state with the same underlying system state.
+     * This is used when spawning operations that need their own writer state instance
+     * while sharing the same system state.
+     *
+     * @return A unique pointer to the cloned writer state.
+     */
+    [[nodiscard]] virtual std::unique_ptr<WriterStateInterface>
+    clone() const = 0;
+};
+
+/**
+ * @brief Implementation of WriterStateInterface that manages ETL writer state.
+ *
+ * This class coordinates with SystemState to manage whether the ETL process
+ * is actively writing to the database. It provides methods to query the current
+ * writing state and request transitions between writing and non-writing states.
+ */
+class WriterState : public WriterStateInterface {
+private:
+    std::shared_ptr<SystemState> systemState_; /**< @brief Shared system state for ETL coordination */
+
+public:
+    /**
+     * @brief Construct a WriterState with the given system state.
+     * @param state Shared pointer to the system state for coordination
+     */
+    WriterState(std::shared_ptr<SystemState> state);
+
+    bool
+    isReadOnly() const override;
+
+    /**
+     * @brief Check if the ETL process is currently writing to the database.
+     * @return true if the process is writing, false otherwise
+     */
+    bool
+    isWriting() const override;
+
+    /**
+     * @brief Request to start writing to the database.
+     *
+     * If already writing, this method does nothing. Otherwise, it sets the
+     * shouldTakeoverWriting flag in the system state to signal the request.
+     */
+    void
+    startWriting() override;
+
+    /**
+     * @brief Request to stop writing to the database.
+     *
+     * If not currently writing, this method does nothing. Otherwise, it sets the
+     * shouldGiveUpWriter flag in the system state to signal the request.
+     */
+    void
+    giveUpWriting() override;
+
+    /**
+     * @brief Switch the cluster to the fallback writer decision mechanism.
+     *
+     * Sets the isWriterDecidingFallback flag in the system state, which will be
+     * propagated to other nodes in the cluster through the ClioNode DbRole::Fallback state.
+     */
+    void
+    setWriterDecidingFallback() override;
+
+    /**
+     * @brief Check if the cluster is using the fallback writer decision mechanism.
+     *
+     * @return true if the cluster has switched to fallback mode, false otherwise
+     */
+    bool
+    isFallback() const override;
+
+    /**
+     * @brief Whether clio is still loading cache after startup.
+     *
+     * @return true if clio is still loading cache, false otherwise.
+     */
+    bool
+    isLoadingCache() const override;
+
+    /**
+     * @brief Create a clone of this writer state.
+     *
+     * Creates a new WriterState instance sharing the same system state.
+     *
+     * @return A unique pointer to the cloned writer state.
+     */
+    std::unique_ptr<WriterStateInterface>
+    clone() const override;
+};
+
+}  // namespace etl

src/etl/impl/LedgerPublisher.hpp

@@ -45,6 +45,7 @@
 #include <xrpl/protocol/Serializer.h>
 
 #include <algorithm>
+#include <atomic>
 #include <chrono>
 #include <cstddef>
 #include <cstdint>
@@ -76,6 +77,8 @@ class LedgerPublisher : public LedgerPublisherInterface {
 
     util::async::AnyStrand publishStrand_;
 
+    std::atomic_bool stop_{false};
+
     std::shared_ptr<BackendInterface> backend_;
     std::shared_ptr<feed::SubscriptionManagerInterface> subscriptions_;
     std::reference_wrapper<SystemState const> state_;  // shared state for ETL
@@ -125,7 +128,7 @@ public:
     {
         LOG(log_.info()) << "Attempting to publish ledger = " << ledgerSequence;
         size_t numAttempts = 0;
-        while (not state_.get().isStopping) {
+        while (not stop_) {
             auto range = backend_->hardFetchLedgerRangeNoThrow();
 
             if (!range || range->maxSequence < ledgerSequence) {
@@ -258,6 +261,18 @@ public:
         return *lastPublishedSequence_.lock();
     }
 
+    /**
+     * @brief Stops publishing
+     *
+     * @note This is a basic implementation to satisfy tests. This will be improved in
+     * https://github.com/XRPLF/clio/issues/2833
+     */
+    void
+    stop()
+    {
+        stop_ = true;
+    }
+
 private:
     void
     setLastClose(std::chrono::time_point<ripple::NetClock> lastCloseTime)

src/etl/impl/Loading.cpp

@@ -75,7 +75,10 @@ Loader::load(model::LedgerData const& data)
                              << "; took " << duration << "ms";
 
             if (not success) {
-                state_->writeConflict = true;
+                // Write conflict detected - another node wrote to the database
+                // This triggers the fallback mechanism and stops this node from writing
+                state_->writeCommandSignal(SystemState::WriteCommand::StopWriting);
+                state_->isWriterDecidingFallback = true;
                 LOG(log_.warn()) << "Another node wrote a ledger into the DB - we have a write conflict";
                 return std::unexpected(LoaderError::WriteConflict);
             }

src/util/Channel.hpp

@@ -19,6 +19,8 @@
 
 #pragma once
 
+#include "util/async/Concepts.hpp"
+
 #include <boost/asio/any_io_executor.hpp>
 #include <boost/asio/experimental/channel.hpp>
 #include <boost/asio/experimental/concurrent_channel.hpp>
@@ -42,15 +44,36 @@ struct ChannelInstantiated;
 }  // namespace detail
 #endif
 
+/**
+ * @brief Specifies the producer concurrency model for a Channel.
+ */
+enum class ProducerType {
+    Single, /**< Only one Sender can exist (non-copyable). Uses direct Guard ownership for zero overhead. */
+    Multi   /**< Multiple Senders can exist (copyable). Uses shared_ptr<Guard> for shared ownership. */
+};
+
+/**
+ * @brief Specifies the consumer concurrency model for a Channel.
+ */
+enum class ConsumerType {
+    Single, /**< Only one Receiver can exist (non-copyable). Uses direct Guard ownership for zero overhead. */
+    Multi   /**< Multiple Receivers can exist (copyable). Uses shared_ptr<Guard> for shared ownership. */
+};
+
 /**
  * @brief Represents a go-like channel, a multi-producer (Sender) multi-consumer (Receiver) thread-safe data pipe.
  * @note Use INSTANTIATE_CHANNEL_FOR_CLANG macro when using this class. See docs at the bottom of the file for more
  * details.
  *
  * @tparam T The type of data the channel transfers
+ * @tparam P ProducerType::Multi (default) for multi-producer or ProducerType::Single for single-producer
+ * @tparam C ConsumerType::Multi (default) for multi-consumer or ConsumerType::Single for single-consumer
  */
-template <typename T>
+template <typename T, ProducerType P = ProducerType::Multi, ConsumerType C = ConsumerType::Multi>
 class Channel {
+    static constexpr bool kIS_MULTI_PRODUCER = (P == ProducerType::Multi);
+    static constexpr bool kIS_MULTI_CONSUMER = (C == ConsumerType::Multi);
+
 private:
     class ControlBlock {
         using InternalChannelType = boost::asio::experimental::concurrent_channel<void(boost::system::error_code, T)>;
@@ -58,7 +81,16 @@ private:
         InternalChannelType ch_;
 
     public:
-        ControlBlock(auto&& context, std::size_t capacity) : executor_(context.get_executor()), ch_(context, capacity)
+        template <typename ContextType>
+            requires(not async::SomeExecutionContext<ContextType>)
+        ControlBlock(ContextType&& context, std::size_t capacity)
+            : executor_(context.get_executor()), ch_(context, capacity)
+        {
+        }
+
+        template <async::SomeExecutionContext ContextType>
+        ControlBlock(ContextType&& context, std::size_t capacity)
+            : executor_(context.getExecutor().get_executor()), ch_(context.getExecutor(), capacity)
         {
         }
 
@@ -101,30 +133,54 @@ private:
         }
     };
 
+public:
     /**
      * @brief The sending end of a channel.
      *
-     * Sender is copyable and movable. The channel remains open as long as at least one Sender exists.
+     * Sender is movable. For multi-producer channels, Sender is also copyable.
+     * The channel remains open as long as at least one Sender exists.
      * When all Sender instances are destroyed, the channel is closed and receivers will receive std::nullopt.
      */
     class Sender {
         std::shared_ptr<ControlBlock> shared_;
-        std::shared_ptr<Guard> guard_;
+        std::conditional_t<kIS_MULTI_PRODUCER, std::shared_ptr<Guard>, Guard> guard_;
+
+        friend class Channel<T, P, C>;
 
-    public:
         /**
          * @brief Constructs a Sender from a shared control block.
          * @param shared The shared control block managing the channel state
          */
         Sender(std::shared_ptr<ControlBlock> shared)
-            : shared_(std::move(shared)), guard_(std::make_shared<Guard>(shared_)) {};
+            : shared_(shared), guard_([shared = std::move(shared)]() {
+                if constexpr (kIS_MULTI_PRODUCER) {
+                    return std::make_shared<Guard>(std::move(shared));
+                } else {
+                    return Guard{std::move(shared)};
+                }
+            }())
+        {
+        }
 
+    public:
         Sender(Sender&&) = default;
-        Sender(Sender const&) = default;
+        Sender(Sender const&)
+            requires kIS_MULTI_PRODUCER
+        = default;
+        Sender(Sender const&)
+            requires(!kIS_MULTI_PRODUCER)
+        = delete;
+
         Sender&
         operator=(Sender&&) = default;
         Sender&
-        operator=(Sender const&) = default;
+        operator=(Sender const&)
+            requires kIS_MULTI_PRODUCER
+        = default;
+        Sender&
+        operator=(Sender const&)
+            requires(!kIS_MULTI_PRODUCER)
+        = delete;
 
         /**
          * @brief Asynchronously sends data through the channel using a coroutine.
@@ -202,27 +258,50 @@ private:
     /**
      * @brief The receiving end of a channel.
      *
-     * Receiver is copyable and movable. Multiple receivers can consume from the same channel concurrently.
+     * Receiver is movable. For multi-consumer channels, Receiver is also copyable.
+     * Multiple receivers can consume from the same multi-consumer channel concurrently.
      * When all Receiver instances are destroyed, the channel is closed and senders will fail to send.
      */
     class Receiver {
         std::shared_ptr<ControlBlock> shared_;
-        std::shared_ptr<Guard> guard_;
+        std::conditional_t<kIS_MULTI_CONSUMER, std::shared_ptr<Guard>, Guard> guard_;
+
+        friend class Channel<T, P, C>;
 
-    public:
         /**
          * @brief Constructs a Receiver from a shared control block.
          * @param shared The shared control block managing the channel state
          */
         Receiver(std::shared_ptr<ControlBlock> shared)
-            : shared_(std::move(shared)), guard_(std::make_shared<Guard>(shared_)) {};
+            : shared_(shared), guard_([shared = std::move(shared)]() {
+                if constexpr (kIS_MULTI_CONSUMER) {
+                    return std::make_shared<Guard>(std::move(shared));
+                } else {
+                    return Guard{std::move(shared)};
+                }
+            }())
+        {
+        }
 
+    public:
         Receiver(Receiver&&) = default;
-        Receiver(Receiver const&) = default;
+        Receiver(Receiver const&)
+            requires kIS_MULTI_CONSUMER
+        = default;
+        Receiver(Receiver const&)
+            requires(!kIS_MULTI_CONSUMER)
+        = delete;
+
         Receiver&
         operator=(Receiver&&) = default;
         Receiver&
-        operator=(Receiver const&) = default;
+        operator=(Receiver const&)
+            requires kIS_MULTI_CONSUMER
+        = default;
+        Receiver&
+        operator=(Receiver const&)
+            requires(!kIS_MULTI_CONSUMER)
+        = delete;
 
         /**
          * @brief Attempts to receive data from the channel without blocking.
@@ -297,7 +376,6 @@ private:
         }
     };
 
-public:
     /**
      * @brief Factory function to create channel components.
      * @param context A supported context type (either io_context or thread_pool)

src/util/Spawn.hpp

@@ -22,6 +22,7 @@
 #include <boost/asio/spawn.hpp>
 #include <boost/asio/strand.hpp>
 
+#include <concepts>
 #include <exception>
 #include <type_traits>
 

src/util/async/Concepts.hpp

@@ -29,6 +29,27 @@
 
 namespace util::async {
 
+/**
+ * @brief Tag type for identifying execution context types.
+ *
+ * Types that inherit from this tag can be detected using the SomeExecutionContext concept.
+ * This allows generic code to differentiate between raw Boost.Asio contexts and wrapped execution contexts.
+ */
+struct ExecutionContextTag {
+    virtual ~ExecutionContextTag() = default;
+};
+
+/**
+ * @brief Concept that identifies types derived from ExecutionContextTag.
+ *
+ * This concept is used to detect custom execution context wrappers (like BasicExecutionContext)
+ * and distinguish them from raw Boost.Asio contexts (io_context, thread_pool, etc.).
+ *
+ * @tparam T The type to check
+ */
+template <typename T>
+concept SomeExecutionContext = std::derived_from<std::remove_cvref_t<T>, ExecutionContextTag>;
+
 /**
  * @brief Specifies the interface for an entity that can be stopped
  */

src/util/async/context/BasicExecutionContext.hpp

@@ -129,7 +129,7 @@ template <
     typename DispatcherType,
     typename TimerContextProvider = impl::SelfContextProvider,
     typename ErrorHandlerType = impl::DefaultErrorHandler>
-class BasicExecutionContext {
+class BasicExecutionContext : public ExecutionContextTag {
     ContextType context_;
 
     /** @cond */
@@ -182,7 +182,7 @@ public:
     /**
      * @brief Stops the underlying thread pool.
      */
-    ~BasicExecutionContext()
+    ~BasicExecutionContext() override
     {
         stop();
     }
@@ -402,6 +402,20 @@ public:
     {
         context_.join();
     }
+
+    /**
+     * @brief Get the underlying executor.
+     *
+     * Provides access to the wrapped executor for cases where the execution context
+     * needs to interact with components that require explicit executor access (like Channel).
+     *
+     * @return Reference to the underlying executor
+     */
+    typename ContextType::Executor&
+    getExecutor()
+    {
+        return context_.getExecutor();
+    }
 };
 
 /**

src/util/async/context/impl/Execution.hpp

@@ -36,17 +36,26 @@ struct SpawnDispatchStrategy {
     {
         auto op = outcome.getOperation();
 
-        util::spawn(
-            ctx.getExecutor(),
-            [outcome = std::forward<OutcomeType>(outcome), fn = std::forward<FnType>(fn)](auto yield) mutable {
-                if constexpr (SomeStoppableOutcome<OutcomeType>) {
-                    auto& stopSource = outcome.getStopSource();
-                    std::invoke(std::forward<decltype(fn)>(fn), outcome, stopSource, stopSource[yield]);
-                } else {
+        if constexpr (SomeStoppableOutcome<OutcomeType>) {
+            util::spawn(
+                ctx.getExecutor(),
+                [outcome = std::forward<OutcomeType>(outcome), fn = std::forward<FnType>(fn)](auto yield) mutable {
+                    if constexpr (SomeStoppableOutcome<OutcomeType>) {
+                        auto& stopSource = outcome.getStopSource();
+                        std::invoke(std::forward<decltype(fn)>(fn), outcome, stopSource, stopSource[yield]);
+                    } else {
+                        std::invoke(std::forward<decltype(fn)>(fn), outcome);
+                    }
+                }
+            );
+        } else {
+            boost::asio::post(
+                ctx.getExecutor(),
+                [outcome = std::forward<OutcomeType>(outcome), fn = std::forward<FnType>(fn)]() mutable {
                     std::invoke(std::forward<decltype(fn)>(fn), outcome);
                 }
-            }
-        );
+            );
+        }
 
         return op;
     }
@@ -55,7 +64,7 @@ struct SpawnDispatchStrategy {
     static void
     post(ContextType& ctx, FnType&& fn)
     {
-        util::spawn(ctx.getExecutor(), [fn = std::forward<FnType>(fn)](auto) mutable {
+        boost::asio::post(ctx.getExecutor(), [fn = std::forward<FnType>(fn)]() mutable {
             std::invoke(std::forward<decltype(fn)>(fn));
         });
     }

tests/common/util/MockBackendTestFixture.hpp

@@ -25,6 +25,7 @@
 #include "util/config/ConfigDefinition.hpp"
 
 #include <gmock/gmock.h>
+#include <gtest/gtest.h>
 
 #include <memory>
 

tests/common/util/MockWriterState.hpp

@@ -0,0 +1,40 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#pragma once
+
+#include "etl/WriterState.hpp"
+
+#include <gmock/gmock.h>
+
+#include <memory>
+
+struct MockWriterStateBase : public etl::WriterStateInterface {
+    MOCK_METHOD(bool, isReadOnly, (), (const, override));
+    MOCK_METHOD(bool, isWriting, (), (const, override));
+    MOCK_METHOD(void, startWriting, (), (override));
+    MOCK_METHOD(void, giveUpWriting, (), (override));
+    MOCK_METHOD(void, setWriterDecidingFallback, (), (override));
+    MOCK_METHOD(bool, isFallback, (), (const, override));
+    MOCK_METHOD(bool, isLoadingCache, (), (const, override));
+    MOCK_METHOD(std::unique_ptr<etl::WriterStateInterface>, clone, (), (const, override));
+};
+
+using MockWriterState = testing::StrictMock<MockWriterStateBase>;
+using NiceMockWriterState = testing::NiceMock<MockWriterStateBase>;

tests/unit/CMakeLists.txt

@@ -21,8 +21,12 @@ target_sources(
           data/impl/LedgerCacheFileTests.cpp
           data/impl/OutputFileTests.cpp
           # Cluster
+          cluster/BackendTests.cpp
           cluster/ClioNodeTests.cpp
           cluster/ClusterCommunicationServiceTests.cpp
+          cluster/MetricsTests.cpp
+          cluster/RepeatedTaskTests.cpp
+          cluster/WriterDeciderTests.cpp
           # ETL
           etl/AmendmentBlockHandlerTests.cpp
           etl/CacheLoaderSettingsTests.cpp
@@ -46,7 +50,9 @@ target_sources(
           etl/SchedulingTests.cpp
           etl/SourceImplTests.cpp
           etl/SubscriptionSourceTests.cpp
+          etl/SystemStateTests.cpp
           etl/TaskManagerTests.cpp
+          etl/WriterStateTests.cpp
           etl/ext/CoreTests.cpp
           etl/ext/CacheTests.cpp
           etl/ext/MPTTests.cpp

tests/unit/app/StopperTests.cpp

@@ -17,6 +17,7 @@
 */
 //==============================================================================
 #include "app/Stopper.hpp"
+#include "cluster/Concepts.hpp"
 #include "util/AsioContextTestFixture.hpp"
 #include "util/MockBackend.hpp"
 #include "util/MockETLService.hpp"
@@ -87,6 +88,10 @@ struct StopperMakeCallbackTest : util::prometheus::WithPrometheus, SyncAsioConte
         MOCK_METHOD(void, waitToFinish, ());
     };
 
+    struct MockClusterCommunicationService : cluster::ClusterCommunicationServiceTag {
+        MOCK_METHOD(void, stop, (), ());
+    };
+
 protected:
     testing::StrictMock<ServerMock> serverMock_;
     testing::StrictMock<MockLoadBalancer> loadBalancerMock_;
@@ -94,6 +99,7 @@ protected:
     testing::StrictMock<MockSubscriptionManager> subscriptionManagerMock_;
     testing::StrictMock<MockBackend> backendMock_{util::config::ClioConfigDefinition{}};
     testing::StrictMock<MockLedgerCacheSaver> cacheSaverMock_;
+    testing::StrictMock<MockClusterCommunicationService> clusterCommunicationServiceMock_;
     boost::asio::io_context ioContextToStop_;
 
     bool
@@ -115,6 +121,7 @@ TEST_F(StopperMakeCallbackTest, makeCallbackTest)
         subscriptionManagerMock_,
         backendMock_,
         cacheSaverMock_,
+        clusterCommunicationServiceMock_,
         ioContextToStop_
     );
 
@@ -122,6 +129,9 @@ TEST_F(StopperMakeCallbackTest, makeCallbackTest)
     EXPECT_CALL(cacheSaverMock_, save).InSequence(s1).WillOnce([this]() { EXPECT_FALSE(isContextStopped()); });
     EXPECT_CALL(serverMock_, stop).InSequence(s1).WillOnce([this]() { EXPECT_FALSE(isContextStopped()); });
     EXPECT_CALL(loadBalancerMock_, stop).InSequence(s2).WillOnce([this]() { EXPECT_FALSE(isContextStopped()); });
+    EXPECT_CALL(clusterCommunicationServiceMock_, stop).InSequence(s1, s2).WillOnce([this]() {
+        EXPECT_FALSE(isContextStopped());
+    });
     EXPECT_CALL(etlServiceMock_, stop).InSequence(s1, s2).WillOnce([this]() { EXPECT_FALSE(isContextStopped()); });
     EXPECT_CALL(subscriptionManagerMock_, stop).InSequence(s1, s2).WillOnce([this]() {
         EXPECT_FALSE(isContextStopped());

tests/unit/cluster/BackendTests.cpp

@@ -0,0 +1,347 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "data/BackendInterface.hpp"
+#include "util/MockBackendTestFixture.hpp"
+#include "util/MockPrometheus.hpp"
+#include "util/MockWriterState.hpp"
+
+#include <boost/asio/thread_pool.hpp>
+#include <boost/json/parse.hpp>
+#include <boost/json/value_to.hpp>
+#include <boost/uuid/random_generator.hpp>
+#include <boost/uuid/uuid.hpp>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <chrono>
+#include <memory>
+#include <semaphore>
+#include <stdexcept>
+#include <string>
+#include <thread>
+#include <utility>
+#include <vector>
+
+using namespace cluster;
+
+struct ClusterBackendTest : util::prometheus::WithPrometheus, MockBackendTestStrict {
+    ~ClusterBackendTest() override
+    {
+        ctx.stop();
+        ctx.join();
+    }
+
+    boost::asio::thread_pool ctx;
+    std::unique_ptr<MockWriterState> writerState = std::make_unique<MockWriterState>();
+    MockWriterState& writerStateRef = *writerState;
+    testing::StrictMock<testing::MockFunction<void(ClioNode::CUuid, std::shared_ptr<Backend::ClusterData const>)>>
+        callbackMock;
+    std::binary_semaphore semaphore{0};
+
+    class SemaphoreReleaseGuard {
+        std::binary_semaphore& semaphore_;
+
+    public:
+        SemaphoreReleaseGuard(std::binary_semaphore& s) : semaphore_(s)
+        {
+        }
+        ~SemaphoreReleaseGuard()
+        {
+            semaphore_.release();
+        }
+    };
+};
+
+TEST_F(ClusterBackendTest, SubscribeToNewState)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(testing::Return(BackendInterface::ClioNodesDataFetchResult{}));
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([this](ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_TRUE(clusterData->has_value());
+            EXPECT_EQ(clusterData->value().size(), 1);
+            auto const& nodeData = clusterData->value().front();
+            EXPECT_EQ(nodeData.uuid, selfId);
+            EXPECT_EQ(nodeData.dbRole, ClioNode::DbRole::ReadOnly);
+            EXPECT_LE(nodeData.updateTime, std::chrono::system_clock::now());
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, Stop)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(testing::Return(BackendInterface::ClioNodesDataFetchResult{}));
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+
+    clusterBackend.run();
+    std::this_thread::sleep_for(std::chrono::milliseconds{20});
+    clusterBackend.stop();
+
+    testing::Mock::VerifyAndClearExpectations(&(*backend_));
+    // Wait to make sure there is no new calls of mockDbBackend
+    std::this_thread::sleep_for(std::chrono::milliseconds{20});
+}
+
+TEST_F(ClusterBackendTest, FetchClioNodesDataThrowsException)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(testing::Throw(std::runtime_error("Database connection failed")));
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([this](ClioNode::CUuid, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_FALSE(clusterData->has_value());
+            EXPECT_EQ(clusterData->error(), "Failed to fetch Clio nodes data");
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, FetchClioNodesDataReturnsDataWithOtherNodes)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    auto const otherUuid = boost::uuids::random_generator{}();
+    auto const otherNodeJson = R"JSON({
+        "db_role": 3,
+        "update_time": "2025-01-15T10:30:00Z"
+    })JSON";
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(
+            testing::Return(
+                BackendInterface::ClioNodesDataFetchResult{
+                    std::vector<std::pair<boost::uuids::uuid, std::string>>{{otherUuid, otherNodeJson}}
+                }
+            )
+        );
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isFallback).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isLoadingCache).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isWriting).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([&](ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_TRUE(clusterData->has_value()) << clusterData->error();
+            EXPECT_EQ(clusterData->value().size(), 2);
+            EXPECT_EQ(selfId, clusterBackend.selfId());
+
+            bool foundSelf = false;
+            bool foundOther = false;
+
+            for (auto const& node : clusterData->value()) {
+                if (*node.uuid == *selfId) {
+                    foundSelf = true;
+                    EXPECT_EQ(node.dbRole, ClioNode::DbRole::NotWriter);
+                } else if (*node.uuid == otherUuid) {
+                    foundOther = true;
+                    EXPECT_EQ(node.dbRole, ClioNode::DbRole::Writer);
+                }
+                EXPECT_LE(node.updateTime, std::chrono::system_clock::now());
+            }
+
+            EXPECT_TRUE(foundSelf);
+            EXPECT_TRUE(foundOther);
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, FetchClioNodesDataReturnsOnlySelfData)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    auto const selfNodeJson = R"JSON({
+        "db_role": 1,
+        "update_time": "2025-01-16T10:30:00Z"
+    })JSON";
+
+    EXPECT_CALL(*backend_, fetchClioNodesData).Times(testing::AtLeast(1)).WillRepeatedly([&]() {
+        return BackendInterface::ClioNodesDataFetchResult{
+            std::vector<std::pair<boost::uuids::uuid, std::string>>{{*clusterBackend.selfId(), selfNodeJson}}
+        };
+    });
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([this](ClioNode::CUuid selfId, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_TRUE(clusterData->has_value());
+            EXPECT_EQ(clusterData->value().size(), 1);
+            auto const& nodeData = clusterData->value().front();
+            EXPECT_EQ(nodeData.uuid, selfId);
+            EXPECT_EQ(nodeData.dbRole, ClioNode::DbRole::ReadOnly);
+            EXPECT_LE(nodeData.updateTime, std::chrono::system_clock::now());
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, FetchClioNodesDataReturnsInvalidJson)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    auto const otherUuid = boost::uuids::random_generator{}();
+    auto const invalidJson = "{ invalid json";
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(
+            testing::Return(
+                BackendInterface::ClioNodesDataFetchResult{
+                    std::vector<std::pair<boost::uuids::uuid, std::string>>{{otherUuid, invalidJson}}
+                }
+            )
+        );
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([this, invalidJson](ClioNode::CUuid, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_FALSE(clusterData->has_value());
+            EXPECT_THAT(clusterData->error(), testing::HasSubstr("Error parsing json from DB"));
+            EXPECT_THAT(clusterData->error(), testing::HasSubstr(invalidJson));
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, FetchClioNodesDataReturnsValidJsonButCannotConvertToClioNode)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    clusterBackend.subscribeToNewState(callbackMock.AsStdFunction());
+
+    auto const otherUuid = boost::uuids::random_generator{}();
+    // Valid JSON but missing required field 'db_role'
+    auto const validJsonMissingField = R"JSON({
+        "update_time": "2025-01-16T10:30:00Z"
+    })JSON";
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(
+            testing::Return(
+                BackendInterface::ClioNodesDataFetchResult{
+                    std::vector<std::pair<boost::uuids::uuid, std::string>>{{otherUuid, validJsonMissingField}}
+                }
+            )
+        );
+    EXPECT_CALL(*backend_, writeNodeMessage).Times(testing::AtLeast(1));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(true));
+    EXPECT_CALL(callbackMock, Call)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([this](ClioNode::CUuid, std::shared_ptr<Backend::ClusterData const> clusterData) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            ASSERT_FALSE(clusterData->has_value());
+            EXPECT_THAT(clusterData->error(), testing::HasSubstr("Error converting json to ClioNode"));
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}
+
+TEST_F(ClusterBackendTest, WriteNodeMessageWritesSelfDataWithRecentTimestampAndDbRole)
+{
+    Backend clusterBackend{
+        ctx, backend_, std::move(writerState), std::chrono::milliseconds(1), std::chrono::milliseconds(1)
+    };
+
+    auto const beforeRun = std::chrono::floor<std::chrono::seconds>(std::chrono::system_clock::now());
+
+    EXPECT_CALL(*backend_, fetchClioNodesData)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly(testing::Return(BackendInterface::ClioNodesDataFetchResult{}));
+    EXPECT_CALL(writerStateRef, isReadOnly).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isFallback).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isLoadingCache).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(writerStateRef, isWriting).Times(testing::AtLeast(1)).WillRepeatedly(testing::Return(false));
+    EXPECT_CALL(*backend_, writeNodeMessage)
+        .Times(testing::AtLeast(1))
+        .WillRepeatedly([&](boost::uuids::uuid const& uuid, std::string message) {
+            SemaphoreReleaseGuard const guard{semaphore};
+            auto const afterWrite = std::chrono::system_clock::now();
+
+            EXPECT_EQ(uuid, *clusterBackend.selfId());
+            auto const json = boost::json::parse(message);
+            auto const node = boost::json::try_value_to<ClioNode>(json);
+            ASSERT_TRUE(node.has_value());
+            EXPECT_EQ(node->dbRole, ClioNode::DbRole::NotWriter);
+            EXPECT_GE(node->updateTime, beforeRun);
+            EXPECT_LE(node->updateTime, afterWrite);
+        });
+
+    clusterBackend.run();
+    semaphore.acquire();
+}

tests/unit/cluster/ClioNodeTests.cpp

@@ -18,6 +18,8 @@
 //==============================================================================
 
 #include "cluster/ClioNode.hpp"
+#include "util/MockWriterState.hpp"
+#include "util/NameGenerator.hpp"
 #include "util/TimeUtils.hpp"
 
 #include <boost/json/object.hpp>
@@ -26,9 +28,11 @@
 #include <boost/json/value_to.hpp>
 #include <boost/uuid/random_generator.hpp>
 #include <boost/uuid/uuid.hpp>
+#include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include <chrono>
+#include <cstdint>
 #include <ctime>
 #include <memory>
 #include <stdexcept>
@@ -44,44 +48,44 @@ struct ClioNodeTest : testing::Test {
 
 TEST_F(ClioNodeTest, Serialization)
 {
-    // Create a ClioNode with test data
     ClioNode const node{
-        .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()()), .updateTime = updateTime
+        .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()()),
+        .updateTime = updateTime,
+        .dbRole = ClioNode::DbRole::Writer
     };
 
-    // Serialize to JSON
     boost::json::value jsonValue;
     EXPECT_NO_THROW(boost::json::value_from(node, jsonValue));
 
-    // Verify JSON structure
     ASSERT_TRUE(jsonValue.is_object()) << jsonValue;
     auto const& obj = jsonValue.as_object();
 
-    // Check update_time exists and is a string
     EXPECT_TRUE(obj.contains("update_time"));
     EXPECT_TRUE(obj.at("update_time").is_string());
+
+    EXPECT_TRUE(obj.contains("db_role"));
+    EXPECT_TRUE(obj.at("db_role").is_number());
+    EXPECT_EQ(obj.at("db_role").as_int64(), static_cast<int64_t>(node.dbRole));
 }
 
 TEST_F(ClioNodeTest, Deserialization)
 {
-    boost::json::value const jsonValue = {{"update_time", updateTimeStr}};
+    boost::json::value const jsonValue = {{"update_time", updateTimeStr}, {"db_role", 1}};
 
-    // Deserialize to ClioNode
-    ClioNode node{.uuid = std::make_shared<boost::uuids::uuid>(), .updateTime = {}};
-    EXPECT_NO_THROW(node = boost::json::value_to<ClioNode>(jsonValue));
+    ClioNode node{
+        .uuid = std::make_shared<boost::uuids::uuid>(), .updateTime = {}, .dbRole = ClioNode::DbRole::ReadOnly
+    };
+    ASSERT_NO_THROW(node = boost::json::value_to<ClioNode>(jsonValue));
 
-    // Verify deserialized data
     EXPECT_NE(node.uuid, nullptr);
     EXPECT_EQ(*node.uuid, boost::uuids::uuid{});
     EXPECT_EQ(node.updateTime, updateTime);
+    EXPECT_EQ(node.dbRole, ClioNode::DbRole::LoadingCache);
 }
 
 TEST_F(ClioNodeTest, DeserializationInvalidTime)
 {
-    // Prepare an invalid time format
     boost::json::value const jsonValue{"update_time", "invalid_format"};
-
-    // Expect an exception during deserialization
     EXPECT_THROW(boost::json::value_to<ClioNode>(jsonValue), std::runtime_error);
 }
 
@@ -93,3 +97,145 @@ TEST_F(ClioNodeTest, DeserializationMissingTime)
     // Expect an exception
     EXPECT_THROW(boost::json::value_to<ClioNode>(jsonValue), std::runtime_error);
 }
+
+struct ClioNodeDbRoleTestBundle {
+    std::string testName;
+    ClioNode::DbRole role;
+};
+
+struct ClioNodeDbRoleTest : ClioNodeTest, testing::WithParamInterface<ClioNodeDbRoleTestBundle> {};
+
+INSTANTIATE_TEST_SUITE_P(
+    AllDbRoles,
+    ClioNodeDbRoleTest,
+    testing::Values(
+        ClioNodeDbRoleTestBundle{.testName = "ReadOnly", .role = ClioNode::DbRole::ReadOnly},
+        ClioNodeDbRoleTestBundle{.testName = "LoadingCache", .role = ClioNode::DbRole::LoadingCache},
+        ClioNodeDbRoleTestBundle{.testName = "NotWriter", .role = ClioNode::DbRole::NotWriter},
+        ClioNodeDbRoleTestBundle{.testName = "Writer", .role = ClioNode::DbRole::Writer},
+        ClioNodeDbRoleTestBundle{.testName = "Fallback", .role = ClioNode::DbRole::Fallback}
+    ),
+    tests::util::kNAME_GENERATOR
+);
+
+TEST_P(ClioNodeDbRoleTest, Serialization)
+{
+    auto const param = GetParam();
+    ClioNode const node{
+        .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()()),
+        .updateTime = updateTime,
+        .dbRole = param.role
+    };
+    auto const jsonValue = boost::json::value_from(node);
+    EXPECT_EQ(jsonValue.as_object().at("db_role").as_int64(), static_cast<int64_t>(param.role));
+}
+
+TEST_P(ClioNodeDbRoleTest, Deserialization)
+{
+    auto const param = GetParam();
+    boost::json::value const jsonValue = {
+        {"update_time", updateTimeStr}, {"db_role", static_cast<int64_t>(param.role)}
+    };
+    auto const node = boost::json::value_to<ClioNode>(jsonValue);
+    EXPECT_EQ(node.dbRole, param.role);
+}
+
+TEST_F(ClioNodeDbRoleTest, DeserializationInvalidDbRole)
+{
+    boost::json::value const jsonValue = {{"update_time", updateTimeStr}, {"db_role", 10}};
+    EXPECT_THROW(boost::json::value_to<ClioNode>(jsonValue), std::runtime_error);
+}
+
+TEST_F(ClioNodeDbRoleTest, DeserializationMissingDbRole)
+{
+    boost::json::value const jsonValue = {{"update_time", updateTimeStr}};
+    EXPECT_THROW(boost::json::value_to<ClioNode>(jsonValue), std::runtime_error);
+}
+
+struct ClioNodeFromTestBundle {
+    std::string testName;
+    bool readOnly;
+    bool fallback;
+    bool loadingCache;
+    bool writing;
+    ClioNode::DbRole expectedRole;
+};
+
+struct ClioNodeFromTest : ClioNodeTest, testing::WithParamInterface<ClioNodeFromTestBundle> {
+    std::shared_ptr<boost::uuids::uuid> uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()());
+
+    MockWriterState writerState;
+};
+
+INSTANTIATE_TEST_SUITE_P(
+    AllWriterStates,
+    ClioNodeFromTest,
+    testing::Values(
+        ClioNodeFromTestBundle{
+            .testName = "ReadOnly",
+            .readOnly = true,
+            .fallback = false,
+            .loadingCache = false,
+            .writing = false,
+            .expectedRole = ClioNode::DbRole::ReadOnly
+        },
+        ClioNodeFromTestBundle{
+            .testName = "Fallback",
+            .readOnly = false,
+            .fallback = true,
+            .loadingCache = false,
+            .writing = false,
+            .expectedRole = ClioNode::DbRole::Fallback
+        },
+        ClioNodeFromTestBundle{
+            .testName = "LoadingCache",
+            .readOnly = false,
+            .fallback = false,
+            .loadingCache = true,
+            .writing = false,
+            .expectedRole = ClioNode::DbRole::LoadingCache
+        },
+        ClioNodeFromTestBundle{
+            .testName = "NotWriterNotReadOnly",
+            .readOnly = false,
+            .fallback = false,
+            .loadingCache = false,
+            .writing = false,
+            .expectedRole = ClioNode::DbRole::NotWriter
+        },
+        ClioNodeFromTestBundle{
+            .testName = "Writer",
+            .readOnly = false,
+            .fallback = false,
+            .loadingCache = false,
+            .writing = true,
+            .expectedRole = ClioNode::DbRole::Writer
+        }
+    ),
+    tests::util::kNAME_GENERATOR
+);
+
+TEST_P(ClioNodeFromTest, FromWriterState)
+{
+    auto const& param = GetParam();
+
+    EXPECT_CALL(writerState, isReadOnly()).WillOnce(testing::Return(param.readOnly));
+    if (not param.readOnly) {
+        EXPECT_CALL(writerState, isFallback()).WillOnce(testing::Return(param.fallback));
+        if (not param.fallback) {
+            EXPECT_CALL(writerState, isLoadingCache()).WillOnce(testing::Return(param.loadingCache));
+            if (not param.loadingCache) {
+                EXPECT_CALL(writerState, isWriting()).WillOnce(testing::Return(param.writing));
+            }
+        }
+    }
+
+    auto const beforeTime = std::chrono::system_clock::now();
+    auto const node = ClioNode::from(uuid, writerState);
+    auto const afterTime = std::chrono::system_clock::now();
+
+    EXPECT_EQ(node.uuid, uuid);
+    EXPECT_EQ(node.dbRole, param.expectedRole);
+    EXPECT_GE(node.updateTime, beforeTime);
+    EXPECT_LE(node.updateTime, afterTime);
+}

tests/unit/cluster/ClusterCommunicationServiceTests.cpp

@@ -22,207 +22,197 @@
 #include "data/BackendInterface.hpp"
 #include "util/MockBackendTestFixture.hpp"
 #include "util/MockPrometheus.hpp"
-#include "util/TimeUtils.hpp"
-#include "util/prometheus/Bool.hpp"
-#include "util/prometheus/Gauge.hpp"
+#include "util/MockWriterState.hpp"
 #include "util/prometheus/Prometheus.hpp"
 
-#include <boost/json/parse.hpp>
+#include <boost/json/object.hpp>
 #include <boost/json/serialize.hpp>
-#include <boost/json/string.hpp>
-#include <boost/json/value.hpp>
 #include <boost/json/value_from.hpp>
-#include <boost/uuid/random_generator.hpp>
 #include <boost/uuid/uuid.hpp>
-#include <boost/uuid/uuid_io.hpp>
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
 #include <algorithm>
+#include <atomic>
 #include <chrono>
-#include <condition_variable>
+#include <cstdint>
 #include <memory>
-#include <mutex>
+#include <semaphore>
 #include <string>
+#include <thread>
 #include <utility>
 #include <vector>
 
 using namespace cluster;
 
-namespace {
-std::vector<ClioNode> const kOTHER_NODES_DATA = {
-    ClioNode{
-        .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()()),
-        .updateTime = util::systemTpFromUtcStr("2015-05-15T12:00:00Z", ClioNode::kTIME_FORMAT).value()
-    },
-    ClioNode{
-        .uuid = std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()()),
-        .updateTime = util::systemTpFromUtcStr("2015-05-15T12:00:01Z", ClioNode::kTIME_FORMAT).value()
-    },
-};
-}  // namespace
+struct ClusterCommunicationServiceTest : util::prometheus::WithPrometheus, MockBackendTest {
+    std::unique_ptr<NiceMockWriterState> writerState = std::make_unique<NiceMockWriterState>();
+    NiceMockWriterState& writerStateRef = *writerState;
 
-struct ClusterCommunicationServiceTest : util::prometheus::WithPrometheus, MockBackendTestStrict {
-    ClusterCommunicationService clusterCommunicationService{
-        backend_,
-        std::chrono::milliseconds{5},
-        std::chrono::milliseconds{9}
-    };
+    static constexpr std::chrono::milliseconds kSHORT_INTERVAL{1};
 
-    util::prometheus::GaugeInt& nodesInClusterMetric = PrometheusService::gaugeInt("cluster_nodes_total_number", {});
-    util::prometheus::Bool isHealthyMetric = PrometheusService::boolMetric("cluster_communication_is_healthy", {});
-
-    std::mutex mtx;
-    std::condition_variable cv;
-
-    void
-    notify()
+    static boost::uuids::uuid
+    makeUuid(uint8_t value)
     {
-        std::unique_lock const lock{mtx};
-        cv.notify_one();
+        boost::uuids::uuid uuid{};
+        std::ranges::fill(uuid, value);
+        return uuid;
     }
 
-    void
-    wait()
+    static ClioNode
+    makeNode(boost::uuids::uuid const& uuid, ClioNode::DbRole role)
     {
-        std::unique_lock lock{mtx};
-        cv.wait_until(lock, std::chrono::steady_clock::now() + std::chrono::milliseconds{100});
+        return ClioNode{
+            .uuid = std::make_shared<boost::uuids::uuid>(uuid),
+            .updateTime = std::chrono::system_clock::now(),
+            .dbRole = role
+        };
+    }
+
+    static std::string
+    nodeToJson(ClioNode const& node)
+    {
+        boost::json::value const v = boost::json::value_from(node);
+        return boost::json::serialize(v);
+    }
+
+    ClusterCommunicationServiceTest()
+    {
+        ON_CALL(writerStateRef, clone()).WillByDefault(testing::Invoke([]() {
+            auto state = std::make_unique<NiceMockWriterState>();
+            ON_CALL(*state, isReadOnly()).WillByDefault(testing::Return(false));
+            ON_CALL(*state, isWriting()).WillByDefault(testing::Return(true));
+            return state;
+        }));
+        ON_CALL(writerStateRef, isReadOnly()).WillByDefault(testing::Return(false));
+        ON_CALL(writerStateRef, isWriting()).WillByDefault(testing::Return(true));
+    }
+
+    static bool
+    waitForSignal(std::binary_semaphore& sem, std::chrono::milliseconds timeout = std::chrono::milliseconds{1000})
+    {
+        return sem.try_acquire_for(timeout);
     }
 };
 
-TEST_F(ClusterCommunicationServiceTest, Write)
+TEST_F(ClusterCommunicationServiceTest, BackendReadsAndWritesData)
 {
-    auto const selfUuid = *clusterCommunicationService.selfUuid();
+    auto const otherUuid = makeUuid(0x02);
+    std::binary_semaphore fetchSemaphore{0};
+    std::binary_semaphore writeSemaphore{0};
 
-    auto const nowStr = util::systemTpToUtcStr(std::chrono::system_clock::now(), ClioNode::kTIME_FORMAT);
-    auto const nowStrPrefix = nowStr.substr(0, nowStr.size() - 3);
+    BackendInterface::ClioNodesDataFetchResult fetchResult{std::vector<std::pair<boost::uuids::uuid, std::string>>{
+        {otherUuid, nodeToJson(makeNode(otherUuid, ClioNode::DbRole::Writer))}
+    }};
 
-    EXPECT_CALL(*backend_, writeNodeMessage(selfUuid, testing::_)).WillOnce([&](auto&&, std::string const& jsonStr) {
-        auto const jv = boost::json::parse(jsonStr);
-        ASSERT_TRUE(jv.is_object());
-        auto const& obj = jv.as_object();
-        ASSERT_TRUE(obj.contains("update_time"));
-        ASSERT_TRUE(obj.at("update_time").is_string());
-        EXPECT_THAT(std::string{obj.at("update_time").as_string()}, testing::StartsWith(nowStrPrefix));
+    ON_CALL(*backend_, fetchClioNodesData).WillByDefault(testing::Invoke([&](auto) {
+        fetchSemaphore.release();
+        return fetchResult;
+    }));
 
-        notify();
-    });
+    ON_CALL(*backend_, writeNodeMessage).WillByDefault(testing::Invoke([&](auto, auto) { writeSemaphore.release(); }));
 
-    clusterCommunicationService.run();
-    wait();
-    // destructor of clusterCommunicationService calls .stop()
+    ClusterCommunicationService service{backend_, std::move(writerState), kSHORT_INTERVAL, kSHORT_INTERVAL};
+
+    service.run();
+
+    EXPECT_TRUE(waitForSignal(fetchSemaphore));
+    EXPECT_TRUE(waitForSignal(writeSemaphore));
+
+    service.stop();
 }
 
-TEST_F(ClusterCommunicationServiceTest, Read_FetchFailed)
+TEST_F(ClusterCommunicationServiceTest, MetricsGetsNewStateFromBackend)
 {
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_CALL(*backend_, writeNodeMessage).Times(2).WillOnce([](auto&&, auto&&) {}).WillOnce([this](auto&&, auto&&) {
-        notify();
-    });
-    EXPECT_CALL(*backend_, fetchClioNodesData).WillRepeatedly([](auto&&) { return std::unexpected{"Failed"}; });
+    auto const otherUuid = makeUuid(0x02);
+    std::binary_semaphore writerActionSemaphore{0};
 
-    clusterCommunicationService.run();
-    wait();
-    // call .stop() manually so that workers exit before expectations are called more times than we want
-    clusterCommunicationService.stop();
+    BackendInterface::ClioNodesDataFetchResult fetchResult{std::vector<std::pair<boost::uuids::uuid, std::string>>{
+        {otherUuid, nodeToJson(makeNode(otherUuid, ClioNode::DbRole::Writer))}
+    }};
 
-    EXPECT_FALSE(isHealthyMetric);
+    ON_CALL(*backend_, fetchClioNodesData).WillByDefault(testing::Invoke([&](auto) { return fetchResult; }));
+
+    ON_CALL(writerStateRef, clone()).WillByDefault(testing::Invoke([&]() mutable {
+        auto state = std::make_unique<NiceMockWriterState>();
+        ON_CALL(*state, startWriting()).WillByDefault(testing::Invoke([&]() { writerActionSemaphore.release(); }));
+        ON_CALL(*state, giveUpWriting()).WillByDefault(testing::Invoke([&]() { writerActionSemaphore.release(); }));
+        return state;
+    }));
+
+    auto& nodesInClusterMetric = PrometheusService::gaugeInt("cluster_nodes_total_number", {});
+    auto isHealthyMetric = PrometheusService::boolMetric("cluster_communication_is_healthy", {});
+
+    ClusterCommunicationService service{backend_, std::move(writerState), kSHORT_INTERVAL, kSHORT_INTERVAL};
+
+    service.run();
+
+    // WriterDecider is called after metrics are updated so we could use it as a signal to stop
+    EXPECT_TRUE(waitForSignal(writerActionSemaphore));
+
+    service.stop();
+
+    EXPECT_EQ(nodesInClusterMetric.value(), 2);
+    EXPECT_TRUE(static_cast<bool>(isHealthyMetric));
 }
 
-TEST_F(ClusterCommunicationServiceTest, Read_FetchThrew)
+TEST_F(ClusterCommunicationServiceTest, WriterDeciderCallsWriterStateMethodsAccordingly)
 {
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_CALL(*backend_, writeNodeMessage).Times(2).WillOnce([](auto&&, auto&&) {}).WillOnce([this](auto&&, auto&&) {
-        notify();
-    });
-    EXPECT_CALL(*backend_, fetchClioNodesData).WillRepeatedly(testing::Throw(data::DatabaseTimeout{}));
+    auto const smallerUuid = makeUuid(0x00);
+    std::binary_semaphore fetchSemaphore{0};
+    std::binary_semaphore writerActionSemaphore{0};
 
-    clusterCommunicationService.run();
-    wait();
-    clusterCommunicationService.stop();
+    BackendInterface::ClioNodesDataFetchResult fetchResult{std::vector<std::pair<boost::uuids::uuid, std::string>>{
+        {smallerUuid, nodeToJson(makeNode(smallerUuid, ClioNode::DbRole::Writer))}
+    }};
 
-    EXPECT_FALSE(isHealthyMetric);
-    EXPECT_FALSE(clusterCommunicationService.clusterData().has_value());
+    ON_CALL(*backend_, fetchClioNodesData).WillByDefault(testing::Invoke([&](auto) {
+        fetchSemaphore.release();
+        return fetchResult;
+    }));
+
+    ON_CALL(*backend_, writeNodeMessage).WillByDefault(testing::Return());
+
+    ON_CALL(writerStateRef, clone()).WillByDefault(testing::Invoke([&]() mutable {
+        auto state = std::make_unique<NiceMockWriterState>();
+        ON_CALL(*state, startWriting()).WillByDefault(testing::Invoke([&]() { writerActionSemaphore.release(); }));
+        ON_CALL(*state, giveUpWriting()).WillByDefault(testing::Invoke([&]() { writerActionSemaphore.release(); }));
+        return state;
+    }));
+
+    ClusterCommunicationService service{backend_, std::move(writerState), kSHORT_INTERVAL, kSHORT_INTERVAL};
+
+    service.run();
+
+    EXPECT_TRUE(waitForSignal(fetchSemaphore));
+    EXPECT_TRUE(waitForSignal(writerActionSemaphore));
+
+    service.stop();
 }
 
-TEST_F(ClusterCommunicationServiceTest, Read_GotInvalidJson)
+TEST_F(ClusterCommunicationServiceTest, StopHaltsBackendOperations)
 {
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_CALL(*backend_, writeNodeMessage).Times(2).WillOnce([](auto&&, auto&&) {}).WillOnce([this](auto&&, auto&&) {
-        notify();
-    });
-    EXPECT_CALL(*backend_, fetchClioNodesData).WillRepeatedly([](auto&&) {
-        return std::vector<std::pair<boost::uuids::uuid, std::string>>{
-            {boost::uuids::random_generator()(), "invalid json"}
-        };
-    });
+    std::atomic<int> backendOperationsCount{0};
+    std::binary_semaphore fetchSemaphore{0};
 
-    clusterCommunicationService.run();
-    wait();
-    clusterCommunicationService.stop();
+    BackendInterface::ClioNodesDataFetchResult fetchResult{std::vector<std::pair<boost::uuids::uuid, std::string>>{}};
 
-    EXPECT_FALSE(isHealthyMetric);
-    EXPECT_FALSE(clusterCommunicationService.clusterData().has_value());
-}
-
-TEST_F(ClusterCommunicationServiceTest, Read_GotInvalidNodeData)
-{
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_CALL(*backend_, writeNodeMessage).Times(2).WillOnce([](auto&&, auto&&) {}).WillOnce([this](auto&&, auto&&) {
-        notify();
-    });
-    EXPECT_CALL(*backend_, fetchClioNodesData).WillRepeatedly([](auto&&) {
-        return std::vector<std::pair<boost::uuids::uuid, std::string>>{{boost::uuids::random_generator()(), "{}"}};
-    });
-
-    clusterCommunicationService.run();
-    wait();
-    clusterCommunicationService.stop();
-
-    EXPECT_FALSE(isHealthyMetric);
-    EXPECT_FALSE(clusterCommunicationService.clusterData().has_value());
-}
-
-TEST_F(ClusterCommunicationServiceTest, Read_Success)
-{
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_EQ(nodesInClusterMetric.value(), 1);
-
-    EXPECT_CALL(*backend_, writeNodeMessage).Times(2).WillOnce([](auto&&, auto&&) {}).WillOnce([this](auto&&, auto&&) {
-        auto const clusterData = clusterCommunicationService.clusterData();
-        ASSERT_TRUE(clusterData.has_value());
-        ASSERT_EQ(clusterData->size(), kOTHER_NODES_DATA.size() + 1);
-        for (auto const& node : kOTHER_NODES_DATA) {
-            auto const it =
-                std::ranges::find_if(*clusterData, [&](ClioNode const& n) { return *(n.uuid) == *(node.uuid); });
-            EXPECT_NE(it, clusterData->cend()) << boost::uuids::to_string(*node.uuid);
-        }
-        auto const selfUuid = clusterCommunicationService.selfUuid();
-        auto const it =
-            std::ranges::find_if(*clusterData, [&selfUuid](ClioNode const& node) { return node.uuid == selfUuid; });
-        EXPECT_NE(it, clusterData->end());
-
-        notify();
-    });
-
-    EXPECT_CALL(*backend_, fetchClioNodesData).WillRepeatedly([this](auto&&) {
-        auto const selfUuid = clusterCommunicationService.selfUuid();
-        std::vector<std::pair<boost::uuids::uuid, std::string>> result = {
-            {*selfUuid, R"JSON({"update_time": "2015-05-15:12:00:00"})JSON"},
-        };
-
-        for (auto const& node : kOTHER_NODES_DATA) {
-            boost::json::value jsonValue;
-            boost::json::value_from(node, jsonValue);
-            result.emplace_back(*node.uuid, boost::json::serialize(jsonValue));
-        }
-        return result;
-    });
-
-    clusterCommunicationService.run();
-    wait();
-    clusterCommunicationService.stop();
-
-    EXPECT_TRUE(isHealthyMetric);
-    EXPECT_EQ(nodesInClusterMetric.value(), 3);
+    ON_CALL(*backend_, fetchClioNodesData).WillByDefault(testing::Invoke([&](auto) {
+        backendOperationsCount++;
+        fetchSemaphore.release();
+        return fetchResult;
+    }));
+    ON_CALL(*backend_, writeNodeMessage).WillByDefault(testing::Invoke([&](auto&&, auto&&) {
+        backendOperationsCount++;
+    }));
+
+    ClusterCommunicationService service{backend_, std::move(writerState), kSHORT_INTERVAL, kSHORT_INTERVAL};
+
+    service.run();
+    EXPECT_TRUE(waitForSignal(fetchSemaphore));
+    service.stop();
+
+    auto const countAfterStop = backendOperationsCount.load();
+    std::this_thread::sleep_for(std::chrono::milliseconds{50});
+    EXPECT_EQ(backendOperationsCount.load(), countAfterStop);
 }

tests/unit/cluster/MetricsTests.cpp

@@ -0,0 +1,189 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "cluster/Metrics.hpp"
+#include "util/MockPrometheus.hpp"
+#include "util/prometheus/Gauge.hpp"
+
+#include <boost/uuid/random_generator.hpp>
+#include <boost/uuid/uuid.hpp>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <chrono>
+#include <expected>
+#include <memory>
+#include <string>
+#include <vector>
+
+using namespace cluster;
+using namespace util::prometheus;
+using namespace testing;
+
+struct MetricsTest : WithMockPrometheus {
+    std::shared_ptr<boost::uuids::uuid> uuid1 =
+        std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()());
+    std::shared_ptr<boost::uuids::uuid> uuid2 =
+        std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()());
+    std::shared_ptr<boost::uuids::uuid> uuid3 =
+        std::make_shared<boost::uuids::uuid>(boost::uuids::random_generator()());
+};
+
+TEST_F(MetricsTest, InitializesMetricsOnConstruction)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics const metrics;
+}
+
+TEST_F(MetricsTest, OnNewStateWithValidClusterData)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics metrics;
+
+    ClioNode const node1{
+        .uuid = uuid1, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::Writer
+    };
+    ClioNode const node2{
+        .uuid = uuid2, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::ReadOnly
+    };
+    ClioNode const node3{
+        .uuid = uuid3, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::NotWriter
+    };
+
+    std::vector<ClioNode> const nodes = {node1, node2, node3};
+    Backend::ClusterData const clusterData = std::expected<std::vector<ClioNode>, std::string>(nodes);
+    auto sharedClusterData = std::make_shared<Backend::ClusterData>(clusterData);
+
+    EXPECT_CALL(isHealthyMock, set(1));
+    EXPECT_CALL(nodesInClusterMock, set(3));
+
+    metrics.onNewState(uuid1, sharedClusterData);
+}
+
+TEST_F(MetricsTest, OnNewStateWithEmptyClusterData)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics metrics;
+
+    std::vector<ClioNode> const nodes = {};
+    Backend::ClusterData const clusterData = std::expected<std::vector<ClioNode>, std::string>(nodes);
+    auto sharedClusterData = std::make_shared<Backend::ClusterData>(clusterData);
+
+    EXPECT_CALL(isHealthyMock, set(1));
+    EXPECT_CALL(nodesInClusterMock, set(0));
+
+    metrics.onNewState(uuid1, sharedClusterData);
+}
+
+TEST_F(MetricsTest, OnNewStateWithFailedClusterData)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics metrics;
+
+    Backend::ClusterData const clusterData =
+        std::expected<std::vector<ClioNode>, std::string>(std::unexpected("Connection failed"));
+    auto sharedClusterData = std::make_shared<Backend::ClusterData>(clusterData);
+
+    EXPECT_CALL(isHealthyMock, set(0));
+    EXPECT_CALL(nodesInClusterMock, set(1));
+
+    metrics.onNewState(uuid1, sharedClusterData);
+}
+
+TEST_F(MetricsTest, OnNewStateWithSingleNode)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics metrics;
+
+    ClioNode const node1{
+        .uuid = uuid1, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::Writer
+    };
+
+    std::vector<ClioNode> const nodes = {node1};
+    Backend::ClusterData const clusterData = std::expected<std::vector<ClioNode>, std::string>(nodes);
+    auto sharedClusterData = std::make_shared<Backend::ClusterData>(clusterData);
+
+    EXPECT_CALL(isHealthyMock, set(1));
+    EXPECT_CALL(nodesInClusterMock, set(1));
+
+    metrics.onNewState(uuid1, sharedClusterData);
+}
+
+TEST_F(MetricsTest, OnNewStateRecoveryFromFailure)
+{
+    auto& nodesInClusterMock = makeMock<GaugeInt>("cluster_nodes_total_number", "");
+    auto& isHealthyMock = makeMock<GaugeInt>("cluster_communication_is_healthy", "");
+
+    EXPECT_CALL(nodesInClusterMock, set(1));
+    EXPECT_CALL(isHealthyMock, set(1));
+
+    Metrics metrics;
+
+    Backend::ClusterData const clusterData1 =
+        std::expected<std::vector<ClioNode>, std::string>(std::unexpected("Connection timeout"));
+    auto sharedClusterData1 = std::make_shared<Backend::ClusterData>(clusterData1);
+
+    EXPECT_CALL(isHealthyMock, set(0));
+    EXPECT_CALL(nodesInClusterMock, set(1));
+
+    metrics.onNewState(uuid1, sharedClusterData1);
+
+    ClioNode const node1{
+        .uuid = uuid1, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::Writer
+    };
+    ClioNode const node2{
+        .uuid = uuid2, .updateTime = std::chrono::system_clock::now(), .dbRole = ClioNode::DbRole::ReadOnly
+    };
+
+    std::vector<ClioNode> const nodes = {node1, node2};
+    Backend::ClusterData const clusterData2 = std::expected<std::vector<ClioNode>, std::string>(nodes);
+    auto sharedClusterData2 = std::make_shared<Backend::ClusterData>(clusterData2);
+
+    EXPECT_CALL(isHealthyMock, set(1));
+    EXPECT_CALL(nodesInClusterMock, set(2));
+
+    metrics.onNewState(uuid2, sharedClusterData2);
+}

tests/unit/cluster/RepeatedTaskTests.cpp

@@ -0,0 +1,223 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/impl/RepeatedTask.hpp"
+#include "util/AsioContextTestFixture.hpp"
+
+#include <boost/asio/io_context.hpp>
+#include <boost/asio/spawn.hpp>
+#include <boost/asio/steady_timer.hpp>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <atomic>
+#include <chrono>
+#include <semaphore>
+#include <thread>
+
+using namespace cluster::impl;
+using namespace testing;
+
+struct RepeatedTaskTest : AsyncAsioContextTest {
+    static constexpr auto kTIMEOUT = std::chrono::seconds{5};
+};
+
+template <typename MockFunctionType>
+struct RepeatedTaskTypedTest : RepeatedTaskTest {
+    std::atomic_int32_t callCount{0};
+    std::binary_semaphore semaphore{0};
+    testing::StrictMock<MockFunctionType> mockFn;
+
+    void
+    expectCalls(int const expectedCalls)
+    {
+        callCount = 0;
+
+        EXPECT_CALL(mockFn, Call).Times(AtLeast(expectedCalls)).WillRepeatedly([this, expectedCalls](auto&&...) {
+            ++callCount;
+            if (callCount >= expectedCalls) {
+                semaphore.release();
+            }
+        });
+    }
+};
+
+namespace {
+
+using TypesToTest = Types<MockFunction<void()>, MockFunction<void(boost::asio::yield_context)>>;
+
+}  // namespace
+
+TYPED_TEST_SUITE(RepeatedTaskTypedTest, TypesToTest);
+
+TYPED_TEST(RepeatedTaskTypedTest, CallsFunctionRepeatedly)
+{
+    RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), this->ctx_);
+
+    this->expectCalls(3);
+
+    task.run(this->mockFn.AsStdFunction());
+
+    EXPECT_TRUE(this->semaphore.try_acquire_for(TestFixture::kTIMEOUT));
+
+    task.stop();
+}
+
+TYPED_TEST(RepeatedTaskTypedTest, StopsImmediately)
+{
+    auto const interval = std::chrono::seconds(5);
+    RepeatedTask<boost::asio::io_context> task(interval, this->ctx_);
+
+    task.run(this->mockFn.AsStdFunction());
+
+    std::this_thread::sleep_for(std::chrono::milliseconds(5));
+
+    auto start = std::chrono::steady_clock::now();
+    task.stop();
+    EXPECT_LT(std::chrono::steady_clock::now() - start, interval);
+}
+
+TYPED_TEST(RepeatedTaskTypedTest, MultipleStops)
+{
+    RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), this->ctx_);
+
+    this->expectCalls(3);
+
+    task.run(this->mockFn.AsStdFunction());
+
+    EXPECT_TRUE(this->semaphore.try_acquire_for(TestFixture::kTIMEOUT));
+
+    task.stop();
+    task.stop();
+    task.stop();
+}
+
+TYPED_TEST(RepeatedTaskTypedTest, DestructorStopsTask)
+{
+    this->expectCalls(3);
+
+    {
+        RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), this->ctx_);
+
+        task.run(this->mockFn.AsStdFunction());
+
+        EXPECT_TRUE(this->semaphore.try_acquire_for(TestFixture::kTIMEOUT));
+
+        // Destructor will call stop()
+    }
+
+    auto const countAfterDestruction = this->callCount.load();
+
+    // Wait a bit - no more calls should happen
+    std::this_thread::sleep_for(std::chrono::milliseconds(10));
+
+    EXPECT_EQ(this->callCount, countAfterDestruction);
+}
+
+TYPED_TEST(RepeatedTaskTypedTest, StopWithoutRunIsNoOp)
+{
+    RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), this->ctx_);
+
+    // Should not crash or hang
+    task.stop();
+}
+
+TEST_F(RepeatedTaskTest, MultipleTasksRunConcurrently)
+{
+    StrictMock<MockFunction<void()>> mockFn1;
+    StrictMock<MockFunction<void()>> mockFn2;
+
+    RepeatedTask<boost::asio::io_context> task1(std::chrono::milliseconds(1), ctx_);
+    RepeatedTask<boost::asio::io_context> task2(std::chrono::milliseconds(2), ctx_);
+
+    std::atomic_int32_t callCount1{0};
+    std::atomic_int32_t callCount2{0};
+    std::binary_semaphore semaphore1{0};
+    std::binary_semaphore semaphore2{0};
+
+    EXPECT_CALL(mockFn1, Call).Times(AtLeast(10)).WillRepeatedly([&]() {
+        if (++callCount1 >= 10) {
+            semaphore1.release();
+        }
+    });
+
+    EXPECT_CALL(mockFn2, Call).Times(AtLeast(5)).WillRepeatedly([&]() {
+        if (++callCount2 >= 5) {
+            semaphore2.release();
+        }
+    });
+
+    task1.run(mockFn1.AsStdFunction());
+    task2.run(mockFn2.AsStdFunction());
+
+    EXPECT_TRUE(semaphore1.try_acquire_for(kTIMEOUT));
+    EXPECT_TRUE(semaphore2.try_acquire_for(kTIMEOUT));
+
+    task1.stop();
+    task2.stop();
+}
+
+TYPED_TEST(RepeatedTaskTypedTest, TaskStateTransitionsCorrectly)
+{
+    RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), this->ctx_);
+
+    task.stop();  // Should be no-op
+
+    this->expectCalls(3);
+
+    task.run(this->mockFn.AsStdFunction());
+
+    EXPECT_TRUE(this->semaphore.try_acquire_for(TestFixture::kTIMEOUT));
+
+    task.stop();
+
+    // Stop again should be no-op
+    task.stop();
+}
+
+TEST_F(RepeatedTaskTest, FunctionCanAccessYieldContext)
+{
+    StrictMock<MockFunction<void(boost::asio::yield_context)>> mockFn;
+    std::atomic_bool yieldContextUsed = false;
+    std::binary_semaphore semaphore{0};
+
+    RepeatedTask<boost::asio::io_context> task(std::chrono::milliseconds(1), ctx_);
+
+    EXPECT_CALL(mockFn, Call).Times(AtLeast(1)).WillRepeatedly([&](boost::asio::yield_context yield) {
+        if (yieldContextUsed)
+            return;
+
+        // Use the yield context to verify it's valid
+        boost::asio::steady_timer timer(yield.get_executor());
+        timer.expires_after(std::chrono::milliseconds(1));
+        boost::system::error_code ec;
+        timer.async_wait(yield[ec]);
+        EXPECT_FALSE(ec) << ec.message();
+        yieldContextUsed = true;
+        semaphore.release();
+    });
+
+    task.run(mockFn.AsStdFunction());
+
+    EXPECT_TRUE(semaphore.try_acquire_for(kTIMEOUT));
+
+    task.stop();
+
+    EXPECT_TRUE(yieldContextUsed);
+}

tests/unit/cluster/WriterDeciderTests.cpp

@@ -0,0 +1,314 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "cluster/Backend.hpp"
+#include "cluster/ClioNode.hpp"
+#include "cluster/WriterDecider.hpp"
+#include "util/MockWriterState.hpp"
+
+#include <boost/asio/thread_pool.hpp>
+#include <boost/uuid/uuid.hpp>
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <algorithm>
+#include <chrono>
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+using namespace cluster;
+
+enum class ExpectedAction { StartWriting, GiveUpWriting, NoAction, SetFallback };
+
+struct WriterDeciderTestParams {
+    std::string testName;
+    uint8_t selfUuidValue;
+    std::vector<std::pair<uint8_t, ClioNode::DbRole>> nodes;
+    ExpectedAction expectedAction;
+    bool useEmptyClusterData = false;
+};
+
+struct WriterDeciderTest : testing::TestWithParam<WriterDeciderTestParams> {
+    ~WriterDeciderTest() override
+    {
+        ctx.stop();
+        ctx.join();
+    }
+
+    boost::asio::thread_pool ctx{1};
+    std::unique_ptr<MockWriterState> writerState = std::make_unique<MockWriterState>();
+    MockWriterState& writerStateRef = *writerState;
+
+    static ClioNode
+    makeNode(boost::uuids::uuid const& uuid, ClioNode::DbRole role)
+    {
+        return ClioNode{
+            .uuid = std::make_shared<boost::uuids::uuid>(uuid),
+            .updateTime = std::chrono::system_clock::now(),
+            .dbRole = role
+        };
+    }
+
+    static boost::uuids::uuid
+    makeUuid(uint8_t value)
+    {
+        boost::uuids::uuid uuid{};
+        std::ranges::fill(uuid, value);
+        return uuid;
+    }
+};
+
+TEST_P(WriterDeciderTest, WriterSelection)
+{
+    auto const& params = GetParam();
+
+    auto const selfUuid = makeUuid(params.selfUuidValue);
+
+    WriterDecider decider{ctx, std::move(writerState)};
+
+    auto clonedState = std::make_unique<MockWriterState>();
+
+    // Set up expectations based on expected action
+    switch (params.expectedAction) {
+        case ExpectedAction::StartWriting:
+            EXPECT_CALL(*clonedState, startWriting());
+            EXPECT_CALL(writerStateRef, clone()).WillOnce(testing::Return(testing::ByMove(std::move(clonedState))));
+            break;
+        case ExpectedAction::GiveUpWriting:
+            EXPECT_CALL(*clonedState, giveUpWriting());
+            EXPECT_CALL(writerStateRef, clone()).WillOnce(testing::Return(testing::ByMove(std::move(clonedState))));
+            break;
+        case ExpectedAction::SetFallback:
+            EXPECT_CALL(*clonedState, setWriterDecidingFallback());
+            EXPECT_CALL(writerStateRef, clone()).WillOnce(testing::Return(testing::ByMove(std::move(clonedState))));
+            break;
+        case ExpectedAction::NoAction:
+            if (not params.useEmptyClusterData) {
+                // For all-ReadOnly case, we still clone but don't call any action
+                EXPECT_CALL(writerStateRef, clone()).WillOnce(testing::Return(testing::ByMove(std::move(clonedState))));
+            }
+            // For empty cluster data, clone is never called
+            break;
+    }
+
+    std::shared_ptr<Backend::ClusterData> clusterData;
+    ClioNode::CUuid selfIdPtr;
+
+    if (params.useEmptyClusterData) {
+        clusterData = std::make_shared<Backend::ClusterData>(std::unexpected(std::string("Communication failed")));
+        selfIdPtr = std::make_shared<boost::uuids::uuid>(selfUuid);
+    } else {
+        std::vector<ClioNode> nodes;
+        nodes.reserve(params.nodes.size());
+        for (auto const& [uuidValue, role] : params.nodes) {
+            auto node = makeNode(makeUuid(uuidValue), role);
+            if (uuidValue == params.selfUuidValue) {
+                selfIdPtr = node.uuid;  // Use the same shared_ptr as in the node
+            }
+            nodes.push_back(std::move(node));
+        }
+        clusterData = std::make_shared<Backend::ClusterData>(std::move(nodes));
+    }
+
+    decider.onNewState(selfIdPtr, clusterData);
+
+    ctx.join();
+}
+
+INSTANTIATE_TEST_SUITE_P(
+    WriterDeciderTests,
+    WriterDeciderTest,
+    testing::Values(
+        WriterDeciderTestParams{
+            .testName = "SelfNodeIsSelectedAsWriter",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::Writer}, {0x02, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "OtherNodeIsSelectedAsWriter",
+            .selfUuidValue = 0x02,
+            .nodes = {{0x01, ClioNode::DbRole::Writer}, {0x02, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "NodesAreSortedByUUID",
+            .selfUuidValue = 0x02,
+            .nodes =
+                {{0x03, ClioNode::DbRole::Writer}, {0x02, ClioNode::DbRole::Writer}, {0x01, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "FirstNodeAfterReadOnlyIsNotSelf",
+            .selfUuidValue = 0x03,
+            .nodes =
+                {{0x01, ClioNode::DbRole::ReadOnly},
+                 {0x02, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "FirstNodeAfterReadOnlyIsSelf",
+            .selfUuidValue = 0x02,
+            .nodes =
+                {{0x01, ClioNode::DbRole::ReadOnly},
+                 {0x02, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "AllNodesReadOnlyGiveUpWriting",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::ReadOnly}, {0x02, ClioNode::DbRole::ReadOnly}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "EmptyClusterDataNoActionTaken",
+            .selfUuidValue = 0x01,
+            .nodes = {},
+            .expectedAction = ExpectedAction::NoAction,
+            .useEmptyClusterData = true
+        },
+        WriterDeciderTestParams{
+            .testName = "SingleNodeClusterSelfIsWriter",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "NotWriterRoleIsSelectedWhenNoWriterRole",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::NotWriter}, {0x02, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "MixedRolesFirstNonReadOnlyIsSelected",
+            .selfUuidValue = 0x03,
+            .nodes =
+                {{0x01, ClioNode::DbRole::ReadOnly},
+                 {0x02, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::NotWriter},
+                 {0x04, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "ShuffledNodesAreSortedCorrectly",
+            .selfUuidValue = 0x04,
+            .nodes =
+                {{0x04, ClioNode::DbRole::Writer},
+                 {0x01, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::Writer},
+                 {0x02, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "ShuffledNodesWithReadOnlySelfIsSelected",
+            .selfUuidValue = 0x03,
+            .nodes =
+                {{0x05, ClioNode::DbRole::Writer},
+                 {0x01, ClioNode::DbRole::ReadOnly},
+                 {0x04, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::Writer},
+                 {0x02, ClioNode::DbRole::ReadOnly}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "SelfIsFallbackNoActionTaken",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::Fallback}, {0x02, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::NoAction
+        },
+        WriterDeciderTestParams{
+            .testName = "OtherNodeIsFallbackSetsFallbackMode",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::Writer}, {0x02, ClioNode::DbRole::Fallback}},
+            .expectedAction = ExpectedAction::SetFallback
+        },
+        WriterDeciderTestParams{
+            .testName = "SelfIsReadOnlyOthersAreFallbackGiveUpWriting",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::ReadOnly}, {0x02, ClioNode::DbRole::Fallback}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "MultipleFallbackNodesSelfNotFallbackSetsFallback",
+            .selfUuidValue = 0x03,
+            .nodes =
+                {{0x01, ClioNode::DbRole::Fallback},
+                 {0x02, ClioNode::DbRole::Fallback},
+                 {0x03, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::SetFallback
+        },
+        WriterDeciderTestParams{
+            .testName = "MixedRolesWithOneFallbackSetsFallback",
+            .selfUuidValue = 0x02,
+            .nodes =
+                {{0x01, ClioNode::DbRole::Writer},
+                 {0x02, ClioNode::DbRole::NotWriter},
+                 {0x03, ClioNode::DbRole::Fallback},
+                 {0x04, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::SetFallback
+        },
+        WriterDeciderTestParams{
+            .testName = "SelfIsLoadingCacheOtherIsWriter",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::LoadingCache}, {0x02, ClioNode::DbRole::Writer}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "OtherNodeIsLoadingCacheSkipToNextWriter",
+            .selfUuidValue = 0x02,
+            .nodes =
+                {{0x01, ClioNode::DbRole::LoadingCache},
+                 {0x02, ClioNode::DbRole::Writer},
+                 {0x03, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "AllNodesLoadingCacheNoActionTaken",
+            .selfUuidValue = 0x01,
+            .nodes = {{0x01, ClioNode::DbRole::LoadingCache}, {0x02, ClioNode::DbRole::LoadingCache}},
+            .expectedAction = ExpectedAction::NoAction
+        },
+        WriterDeciderTestParams{
+            .testName = "MixedWithLoadingCacheReadOnlyFirstNonReadOnlyNonLoadingCacheSelected",
+            .selfUuidValue = 0x03,
+            .nodes =
+                {{0x01, ClioNode::DbRole::ReadOnly},
+                 {0x02, ClioNode::DbRole::LoadingCache},
+                 {0x03, ClioNode::DbRole::Writer},
+                 {0x04, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::StartWriting
+        },
+        WriterDeciderTestParams{
+            .testName = "LoadingCacheBeforeWriterSkipsLoadingCache",
+            .selfUuidValue = 0x04,
+            .nodes =
+                {{0x01, ClioNode::DbRole::LoadingCache},
+                 {0x02, ClioNode::DbRole::LoadingCache},
+                 {0x03, ClioNode::DbRole::Writer},
+                 {0x04, ClioNode::DbRole::NotWriter}},
+            .expectedAction = ExpectedAction::GiveUpWriting
+        }
+    ),
+    [](testing::TestParamInfo<WriterDeciderTestParams> const& info) { return info.param.testName; }
+);

tests/unit/data/AmendmentCenterTests.cpp

@@ -32,8 +32,8 @@
 #include <xrpl/protocol/Indexes.h>
 
 #include <algorithm>
+#include <functional>
 #include <optional>
-#include <stdexcept>
 #include <string>
 #include <vector>
 

tests/unit/etl/ETLServiceTests.cpp

@@ -216,6 +216,10 @@ protected:
     std::shared_ptr<testing::NiceMock<MockMonitorProvider>> monitorProvider_ =
         std::make_shared<testing::NiceMock<MockMonitorProvider>>();
     std::shared_ptr<etl::SystemState> systemState_ = std::make_shared<etl::SystemState>();
+    testing::StrictMock<testing::MockFunction<void(etl::SystemState::WriteCommand)>> mockWriteSignalCommandCallback_;
+    boost::signals2::scoped_connection writeCommandConnection_{
+        systemState_->writeCommandSignal.connect(mockWriteSignalCommandCallback_.AsStdFunction())
+    };
 
     etl::ETLService service_{
         ctx_,
@@ -300,6 +304,7 @@ TEST_F(ETLServiceTests, RunWithEmptyDatabase)
     auto mockTaskManager = std::make_unique<testing::NiceMock<MockTaskManager>>();
     auto& mockTaskManagerRef = *mockTaskManager;
     auto ledgerData = createTestData(kSEQ);
+    EXPECT_TRUE(systemState_->isLoadingCache);
 
     testing::Sequence const s;
     EXPECT_CALL(*backend_, hardFetchLedgerRange).InSequence(s).WillOnce(testing::Return(std::nullopt));
@@ -308,25 +313,61 @@ TEST_F(ETLServiceTests, RunWithEmptyDatabase)
     EXPECT_CALL(*balancer_, loadInitialLedger(kSEQ, testing::_, testing::_))
         .WillOnce(testing::Return(std::vector<std::string>{}));
     EXPECT_CALL(*loader_, loadInitialLedger).WillOnce(testing::Return(ripple::LedgerHeader{}));
-    EXPECT_CALL(*backend_, hardFetchLedgerRange)
-        .InSequence(s)
-        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    // In syncCacheWithDb()
+    EXPECT_CALL(*backend_, hardFetchLedgerRange).Times(2).InSequence(s).WillRepeatedly([this]() {
+        backend_->cache().update({}, kSEQ, false);
+        return data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ};
+    });
     EXPECT_CALL(mockTaskManagerRef, run);
-    EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_))
-        .WillOnce(testing::Return(std::unique_ptr<etl::TaskManagerInterface>(mockTaskManager.release())));
-    EXPECT_CALL(*monitorProvider_, make(testing::_, testing::_, testing::_, testing::_, testing::_))
-        .WillOnce([](auto, auto, auto, auto, auto) { return std::make_unique<testing::NiceMock<MockMonitor>>(); });
+    EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_)).WillOnce([&](auto&&...) {
+        EXPECT_FALSE(systemState_->isLoadingCache);
+        return std::unique_ptr<etl::TaskManagerInterface>(mockTaskManager.release());
+    });
+    EXPECT_CALL(*monitorProvider_, make(testing::_, testing::_, testing::_, kSEQ + 1, testing::_))
+        .WillOnce([this](auto, auto, auto, auto, auto) {
+            EXPECT_TRUE(systemState_->isLoadingCache);
+            return std::make_unique<testing::NiceMock<MockMonitor>>();
+        });
 
     service_.run();
 }
 
 TEST_F(ETLServiceTests, RunWithPopulatedDatabase)
 {
+    EXPECT_TRUE(systemState_->isLoadingCache);
+    backend_->cache().update({}, kSEQ, false);
     EXPECT_CALL(*backend_, hardFetchLedgerRange)
         .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
-    EXPECT_CALL(*monitorProvider_, make).WillOnce([](auto, auto, auto, auto, auto) {
-        return std::make_unique<testing::NiceMock<MockMonitor>>();
-    });
+    EXPECT_CALL(*monitorProvider_, make(testing::_, testing::_, testing::_, kSEQ + 1, testing::_))
+        .WillOnce([this](auto, auto, auto, auto, auto) {
+            EXPECT_TRUE(systemState_->isLoadingCache);
+            return std::make_unique<testing::NiceMock<MockMonitor>>();
+        });
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillRepeatedly(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+}
+
+TEST_F(ETLServiceTests, SyncCacheWithDbBeforeStartingMonitor)
+{
+    EXPECT_TRUE(systemState_->isLoadingCache);
+    backend_->cache().update({}, kSEQ - 2, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+
+    EXPECT_CALL(*backend_, fetchLedgerDiff(kSEQ - 1, testing::_));
+    EXPECT_CALL(*cacheUpdater_, update(kSEQ - 1, std::vector<data::LedgerObject>()))
+        .WillOnce([this](auto const seq, auto&&...) { backend_->cache().update({}, seq, false); });
+    EXPECT_CALL(*backend_, fetchLedgerDiff(kSEQ, testing::_));
+    EXPECT_CALL(*cacheUpdater_, update(kSEQ, std::vector<data::LedgerObject>()))
+        .WillOnce([this](auto const seq, auto&&...) { backend_->cache().update({}, seq, false); });
+
+    EXPECT_CALL(*monitorProvider_, make(testing::_, testing::_, testing::_, kSEQ + 1, testing::_))
+        .WillOnce([this](auto, auto, auto, auto, auto) {
+            EXPECT_TRUE(systemState_->isLoadingCache);
+            return std::make_unique<testing::NiceMock<MockMonitor>>();
+        });
     EXPECT_CALL(*ledgers_, getMostRecent()).WillRepeatedly(testing::Return(kSEQ));
     EXPECT_CALL(*cacheLoader_, load(kSEQ));
 
@@ -364,19 +405,22 @@ TEST_F(ETLServiceTests, HandlesWriteConflictInMonitorSubscription)
     EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
     EXPECT_CALL(mockMonitorRef, run);
 
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
     EXPECT_CALL(*backend_, hardFetchLedgerRange)
-        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+        .Times(2)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
     EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
     EXPECT_CALL(*cacheLoader_, load(kSEQ));
 
     service_.run();
-    systemState_->writeConflict = true;
+    writeCommandConnection_.disconnect();
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StopWriting);
 
     EXPECT_CALL(*publisher_, publish(kSEQ + 1, testing::_, testing::_));
     ASSERT_TRUE(capturedCallback);
     capturedCallback(kSEQ + 1);
 
-    EXPECT_FALSE(systemState_->writeConflict);
     EXPECT_FALSE(systemState_->isWriting);
 }
 
@@ -397,8 +441,11 @@ TEST_F(ETLServiceTests, NormalFlowInMonitorSubscription)
     EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
     EXPECT_CALL(mockMonitorRef, run);
 
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
     EXPECT_CALL(*backend_, hardFetchLedgerRange)
-        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+        .Times(2)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
     EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
     EXPECT_CALL(*cacheLoader_, load(kSEQ));
 
@@ -424,13 +471,19 @@ TEST_F(ETLServiceTests, AttemptTakeoverWriter)
         return std::move(mockMonitor);
     });
 
-    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence);
+    std::function<void(uint32_t)> onNewSeqCallback;
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence).WillOnce([&onNewSeqCallback](auto cb) {
+        onNewSeqCallback = std::move(cb);
+        return boost::signals2::scoped_connection{};
+    });
     EXPECT_CALL(mockMonitorRef, subscribeToDbStalled).WillOnce([&capturedDbStalledCallback](auto callback) {
         capturedDbStalledCallback = callback;
         return boost::signals2::scoped_connection{};
     });
     EXPECT_CALL(mockMonitorRef, run);
 
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
     EXPECT_CALL(*backend_, hardFetchLedgerRange)
         .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
     EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
@@ -447,10 +500,14 @@ TEST_F(ETLServiceTests, AttemptTakeoverWriter)
     EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_))
         .WillOnce(testing::Return(std::move(mockTaskManager)));
 
-    ASSERT_TRUE(capturedDbStalledCallback);
-    capturedDbStalledCallback();
+    EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StartWriting));
 
-    EXPECT_TRUE(systemState_->isWriting);  // should attempt to become writer
+    ASSERT_TRUE(capturedDbStalledCallback);
+    EXPECT_FALSE(systemState_->isWriting);  // will attempt to become writer after new sequence appears but not yet
+    EXPECT_FALSE(systemState_->isWriterDecidingFallback);
+    capturedDbStalledCallback();
+    EXPECT_TRUE(systemState_->isWriting);                 // should attempt to become writer
+    EXPECT_TRUE(systemState_->isWriterDecidingFallback);  // fallback mode activated
 }
 
 TEST_F(ETLServiceTests, GiveUpWriterAfterWriteConflict)
@@ -470,22 +527,25 @@ TEST_F(ETLServiceTests, GiveUpWriterAfterWriteConflict)
     EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
     EXPECT_CALL(mockMonitorRef, run);
 
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
     EXPECT_CALL(*backend_, hardFetchLedgerRange)
-        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+        .Times(2)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
     EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
     EXPECT_CALL(*cacheLoader_, load(kSEQ));
 
     service_.run();
     systemState_->isWriting = true;
-    systemState_->writeConflict = true;  // got a write conflict along the way
+    writeCommandConnection_.disconnect();
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StopWriting);
 
     EXPECT_CALL(*publisher_, publish(kSEQ + 1, testing::_, testing::_));
 
     ASSERT_TRUE(capturedCallback);
     capturedCallback(kSEQ + 1);
 
-    EXPECT_FALSE(systemState_->isWriting);      // gives up writing
-    EXPECT_FALSE(systemState_->writeConflict);  // and removes write conflict flag
+    EXPECT_FALSE(systemState_->isWriting);  // gives up writing
 }
 
 TEST_F(ETLServiceTests, CancelledLoadInitialLedger)
@@ -539,3 +599,327 @@ TEST_F(ETLServiceTests, RunStopsIfInitialLoadIsCancelledByBalancer)
     EXPECT_FALSE(service_.isAmendmentBlocked());
     EXPECT_FALSE(service_.isCorruptionDetected());
 }
+
+TEST_F(ETLServiceTests, DbStalledDoesNotTriggerSignalWhenStrictReadonly)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void()> capturedDbStalledCallback;
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence);
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled).WillOnce([&capturedDbStalledCallback](auto callback) {
+        capturedDbStalledCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isStrictReadonly = true;  // strict readonly mode
+    systemState_->isWriting = false;
+
+    // No signal should be emitted because node is in strict readonly mode
+    // But fallback flag should still be set
+
+    ASSERT_TRUE(capturedDbStalledCallback);
+    EXPECT_FALSE(systemState_->isWriterDecidingFallback);
+    capturedDbStalledCallback();
+    EXPECT_TRUE(systemState_->isWriterDecidingFallback);  // fallback mode activated even in readonly
+}
+
+TEST_F(ETLServiceTests, DbStalledDoesNotTriggerSignalWhenAlreadyWriting)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void()> capturedDbStalledCallback;
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence);
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled).WillOnce([&capturedDbStalledCallback](auto callback) {
+        capturedDbStalledCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isStrictReadonly = false;
+    systemState_->isWriting = true;  // already writing
+
+    // No signal should be emitted because node is already writing
+    // But fallback flag should still be set
+
+    ASSERT_TRUE(capturedDbStalledCallback);
+    EXPECT_FALSE(systemState_->isWriterDecidingFallback);
+    capturedDbStalledCallback();
+    EXPECT_TRUE(systemState_->isWriterDecidingFallback);  // fallback mode activated
+}
+
+TEST_F(ETLServiceTests, CacheUpdatesDependOnActualCacheState_WriterMode)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void(uint32_t)> capturedCallback;
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence).WillOnce([&capturedCallback](auto callback) {
+        capturedCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set cache to be in sync with DB initially to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isWriting = true;  // In writer mode
+
+    // Simulate cache is behind (e.g., update failed previously)
+    // Cache latestLedgerSequence returns kSEQ (behind the new seq kSEQ + 1)
+    std::vector<data::LedgerObject> const emptyObjs = {};
+    backend_->cache().update(emptyObjs, kSEQ);  // Set cache to kSEQ
+
+    std::vector<data::LedgerObject> const dummyDiff = {};
+    EXPECT_CALL(*backend_, fetchLedgerDiff(kSEQ + 1, testing::_)).WillOnce(testing::Return(dummyDiff));
+
+    // Cache should be updated even though we're in writer mode
+    EXPECT_CALL(*cacheUpdater_, update(kSEQ + 1, testing::A<std::vector<data::LedgerObject> const&>()));
+
+    EXPECT_CALL(*publisher_, publish(kSEQ + 1, testing::_, testing::_));
+
+    ASSERT_TRUE(capturedCallback);
+    capturedCallback(kSEQ + 1);
+}
+
+TEST_F(ETLServiceTests, OnlyCacheUpdatesWhenBackendIsCurrent)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void(uint32_t)> capturedCallback;
+    // Set cache to be in sync with DB initially to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence).WillOnce([&capturedCallback](auto callback) {
+        capturedCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set backend range to be at kSEQ + 1 (already current)
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}))
+        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}))
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ + 1}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isWriting = false;
+
+    // Cache is behind (at kSEQ)
+    std::vector<data::LedgerObject> const emptyObjs = {};
+    backend_->cache().update(emptyObjs, kSEQ);
+
+    std::vector<data::LedgerObject> const dummyDiff = {};
+    EXPECT_CALL(*backend_, fetchLedgerDiff(kSEQ + 1, testing::_)).WillOnce(testing::Return(dummyDiff));
+    EXPECT_CALL(*cacheUpdater_, update(kSEQ + 1, testing::A<std::vector<data::LedgerObject> const&>()));
+
+    EXPECT_CALL(*publisher_, publish(kSEQ + 1, testing::_, testing::_));
+
+    ASSERT_TRUE(capturedCallback);
+    capturedCallback(kSEQ + 1);
+}
+
+TEST_F(ETLServiceTests, NoUpdatesWhenBothCacheAndBackendAreCurrent)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void(uint32_t)> capturedCallback;
+    // Set cache to be in sync with DB initially to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence).WillOnce([&capturedCallback](auto callback) {
+        capturedCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set backend range to be at kSEQ + 1 (already current)
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}))
+        .WillOnce(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}))
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ + 1}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+
+    // Cache is current (at kSEQ + 1)
+    std::vector<data::LedgerObject> const emptyObjs = {};
+    backend_->cache().update(emptyObjs, kSEQ + 1);
+
+    // Neither should be updated
+    EXPECT_CALL(*backend_, fetchLedgerDiff).Times(0);
+    EXPECT_CALL(*cacheUpdater_, update(testing::_, testing::A<std::vector<data::LedgerObject> const&>())).Times(0);
+
+    EXPECT_CALL(*publisher_, publish(kSEQ + 1, testing::_, testing::_));
+
+    ASSERT_TRUE(capturedCallback);
+    capturedCallback(kSEQ + 1);
+}
+
+TEST_F(ETLServiceTests, StopWaitsForWriteCommandHandlersToComplete)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isStrictReadonly = false;
+
+    auto mockTaskManager = std::make_unique<testing::NiceMock<MockTaskManager>>();
+
+    EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StartWriting));
+    EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_))
+        .WillOnce(testing::Return(std::move(mockTaskManager)));
+
+    // Emit a command
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StartWriting);
+
+    // The test context processes operations synchronously, so the handler should have run
+    // Stop should wait for the handler to complete and disconnect the subscription
+    service_.stop();
+
+    // Verify stop() returned, meaning all handlers completed
+    SUCCEED();
+}
+
+TEST_F(ETLServiceTests, WriteConflictIsHandledImmediately_NotDelayed)
+{
+    // This test verifies that write conflicts are handled immediately via signal,
+    // not delayed until the next sequence notification (the old behavior)
+
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+    auto& mockMonitorRef = *mockMonitor;
+    std::function<void(uint32_t)> capturedNewSeqCallback;
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToNewSequence).WillOnce([&capturedNewSeqCallback](auto callback) {
+        capturedNewSeqCallback = callback;
+        return boost::signals2::scoped_connection{};
+    });
+    EXPECT_CALL(mockMonitorRef, subscribeToDbStalled);
+    EXPECT_CALL(mockMonitorRef, run);
+
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isWriting = true;
+
+    // Emit StopWriting signal (simulating write conflict from Loader)
+    EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StopWriting));
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StopWriting);
+
+    // The test context processes operations synchronously, so the handler should have run immediately
+    // Verify that isWriting is immediately set to false
+    EXPECT_FALSE(systemState_->isWriting);
+}
+
+TEST_F(ETLServiceTests, WriteCommandsAreSerializedOnStrand)
+{
+    auto mockMonitor = std::make_unique<testing::NiceMock<MockMonitor>>();
+
+    EXPECT_CALL(*monitorProvider_, make).WillOnce([&mockMonitor](auto, auto, auto, auto, auto) {
+        return std::move(mockMonitor);
+    });
+
+    // Set cache to be in sync with DB to avoid syncCacheWithDb loop
+    backend_->cache().update({}, kSEQ, false);
+    EXPECT_CALL(*backend_, hardFetchLedgerRange)
+        .WillRepeatedly(testing::Return(data::LedgerRange{.minSequence = 1, .maxSequence = kSEQ}));
+    EXPECT_CALL(*ledgers_, getMostRecent()).WillOnce(testing::Return(kSEQ));
+    EXPECT_CALL(*cacheLoader_, load(kSEQ));
+
+    service_.run();
+    systemState_->isStrictReadonly = false;
+    systemState_->isWriting = false;
+
+    auto mockTaskManager1 = std::make_unique<testing::NiceMock<MockTaskManager>>();
+    auto mockTaskManager2 = std::make_unique<testing::NiceMock<MockTaskManager>>();
+
+    // Set up expectations for the sequence of write commands
+    // The signals should be processed in order: StartWriting, StopWriting, StartWriting
+    {
+        testing::InSequence const seq;
+
+        // First StartWriting
+        EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StartWriting));
+        EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_))
+            .WillOnce(testing::Return(std::move(mockTaskManager1)));
+
+        // Then StopWriting
+        EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StopWriting));
+
+        // Finally second StartWriting
+        EXPECT_CALL(mockWriteSignalCommandCallback_, Call(etl::SystemState::WriteCommand::StartWriting));
+        EXPECT_CALL(*taskManagerProvider_, make(testing::_, testing::_, kSEQ + 1, testing::_))
+            .WillOnce(testing::Return(std::move(mockTaskManager2)));
+    }
+
+    // Emit multiple signals rapidly - they should be serialized on the strand
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StartWriting);
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StopWriting);
+    systemState_->writeCommandSignal(etl::SystemState::WriteCommand::StartWriting);
+
+    // The test context processes operations synchronously, so all signals should have been processed
+    // Final state should be writing (last signal was StartWriting)
+    EXPECT_TRUE(systemState_->isWriting);
+}

tests/unit/etl/LedgerPublisherTests.cpp

@@ -216,15 +216,14 @@ TEST_F(ETLLedgerPublisherTest, PublishLedgerHeaderCloseTimeGreaterThanNow)
 TEST_F(ETLLedgerPublisherTest, PublishLedgerSeqStopIsTrue)
 {
     auto dummyState = etl::SystemState{};
-    dummyState.isStopping = true;
     auto publisher = impl::LedgerPublisher(ctx, backend_, mockSubscriptionManagerPtr, dummyState);
+    publisher.stop();
     EXPECT_FALSE(publisher.publish(kSEQ, {}));
 }
 
 TEST_F(ETLLedgerPublisherTest, PublishLedgerSeqMaxAttempt)
 {
     auto dummyState = etl::SystemState{};
-    dummyState.isStopping = false;
     auto publisher = impl::LedgerPublisher(ctx, backend_, mockSubscriptionManagerPtr, dummyState);
 
     static constexpr auto kMAX_ATTEMPT = 2;
@@ -238,7 +237,6 @@ TEST_F(ETLLedgerPublisherTest, PublishLedgerSeqMaxAttempt)
 TEST_F(ETLLedgerPublisherTest, PublishLedgerSeqStopIsFalse)
 {
     auto dummyState = etl::SystemState{};
-    dummyState.isStopping = false;
     auto publisher = impl::LedgerPublisher(ctx, backend_, mockSubscriptionManagerPtr, dummyState);
 
     LedgerRange const range{.minSequence = kSEQ, .maxSequence = kSEQ};

tests/unit/etl/LoadingTests.cpp

@@ -19,6 +19,7 @@
 
 #include "data/Types.hpp"
 #include "etl/InitialLoadObserverInterface.hpp"
+#include "etl/LoaderInterface.hpp"
 #include "etl/Models.hpp"
 #include "etl/RegistryInterface.hpp"
 #include "etl/SystemState.hpp"
@@ -188,3 +189,59 @@ TEST_F(LoadingAssertTest, LoadInitialLedgerHasDataInDB)
 
     EXPECT_CLIO_ASSERT_FAIL({ [[maybe_unused]] auto unused = loader_.loadInitialLedger(data); });
 }
+
+TEST_F(LoadingTests, LoadWriteConflictEmitsStopWritingSignal)
+{
+    state_->isWriting = true;  // writer is active
+    auto const data = createTestData();
+    testing::StrictMock<testing::MockFunction<void(etl::SystemState::WriteCommand)>> mockSignalCallback;
+
+    auto connection = state_->writeCommandSignal.connect(mockSignalCallback.AsStdFunction());
+
+    EXPECT_CALL(*mockRegistryPtr_, dispatch(data));
+    EXPECT_CALL(*backend_, doFinishWrites()).WillOnce(testing::Return(false));  // simulate write conflict
+    EXPECT_CALL(mockSignalCallback, Call(etl::SystemState::WriteCommand::StopWriting));
+
+    EXPECT_FALSE(state_->isWriterDecidingFallback);
+
+    auto result = loader_.load(data);
+    EXPECT_FALSE(result.has_value());
+    EXPECT_EQ(result.error(), etl::LoaderError::WriteConflict);
+    EXPECT_TRUE(state_->isWriterDecidingFallback);
+}
+
+TEST_F(LoadingTests, LoadSuccessDoesNotEmitSignal)
+{
+    state_->isWriting = true;  // writer is active
+    auto const data = createTestData();
+    testing::StrictMock<testing::MockFunction<void(etl::SystemState::WriteCommand)>> mockSignalCallback;
+
+    auto connection = state_->writeCommandSignal.connect(mockSignalCallback.AsStdFunction());
+
+    EXPECT_CALL(*mockRegistryPtr_, dispatch(data));
+    EXPECT_CALL(*backend_, doFinishWrites()).WillOnce(testing::Return(true));  // success
+    // No signal should be emitted on success
+
+    EXPECT_FALSE(state_->isWriterDecidingFallback);
+
+    auto result = loader_.load(data);
+    EXPECT_TRUE(result.has_value());
+    EXPECT_FALSE(state_->isWriterDecidingFallback);
+}
+
+TEST_F(LoadingTests, LoadWhenNotWritingDoesNotCheckConflict)
+{
+    state_->isWriting = false;  // not a writer
+    auto const data = createTestData();
+    testing::StrictMock<testing::MockFunction<void(etl::SystemState::WriteCommand)>> mockSignalCallback;
+
+    auto connection = state_->writeCommandSignal.connect(mockSignalCallback.AsStdFunction());
+
+    EXPECT_CALL(*mockRegistryPtr_, dispatch(data));
+    // doFinishWrites should not be called when not writing
+    EXPECT_CALL(*backend_, doFinishWrites()).Times(0);
+    // No signal should be emitted
+
+    auto result = loader_.load(data);
+    EXPECT_TRUE(result.has_value());
+}

tests/unit/etl/RegistryTests.cpp

@@ -260,7 +260,7 @@ struct RegistryTest : util::prometheus::WithPrometheus {
     }
 
 protected:
-    etl::SystemState state_{};
+    etl::SystemState state_;
 };
 
 }  // namespace

tests/unit/etl/SystemStateTests.cpp

@@ -0,0 +1,73 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2026, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "etl/SystemState.hpp"
+#include "util/MockPrometheus.hpp"
+#include "util/config/ConfigDefinition.hpp"
+#include "util/config/ConfigFileJson.hpp"
+#include "util/config/ConfigValue.hpp"
+#include "util/config/Types.hpp"
+
+#include <boost/json/object.hpp>
+#include <boost/json/parse.hpp>
+#include <fmt/format.h>
+#include <gtest/gtest.h>
+
+#include <memory>
+
+using namespace etl;
+using namespace util::config;
+
+struct SystemStateTest : util::prometheus::WithPrometheus {};
+
+TEST_F(SystemStateTest, InitialValuesAreCorrect)
+{
+    auto state = SystemState{};
+
+    EXPECT_FALSE(state.isStrictReadonly);
+    EXPECT_FALSE(state.isWriting);
+    EXPECT_TRUE(state.isLoadingCache);
+    EXPECT_FALSE(state.isAmendmentBlocked);
+    EXPECT_FALSE(state.isCorruptionDetected);
+    EXPECT_FALSE(state.isWriterDecidingFallback);
+}
+
+struct SystemStateReadOnlyTest : util::prometheus::WithPrometheus, testing::WithParamInterface<bool> {};
+
+TEST_P(SystemStateReadOnlyTest, MakeSystemStateWithReadOnly)
+{
+    auto const readOnlyValue = GetParam();
+    auto const configJson = boost::json::parse(fmt::format(R"JSON({{"read_only": {}}})JSON", readOnlyValue));
+
+    auto config = ClioConfigDefinition{{{"read_only", ConfigValue{ConfigType::Boolean}}}};
+    auto const configFile = ConfigFileJson{configJson.as_object()};
+    auto const errors = config.parse(configFile);
+    ASSERT_FALSE(errors.has_value());
+
+    auto state = SystemState::makeSystemState(config);
+
+    EXPECT_EQ(state->isStrictReadonly, readOnlyValue);
+    EXPECT_FALSE(state->isWriting);
+    EXPECT_TRUE(state->isLoadingCache);
+    EXPECT_FALSE(state->isAmendmentBlocked);
+    EXPECT_FALSE(state->isCorruptionDetected);
+    EXPECT_FALSE(state->isWriterDecidingFallback);
+}
+
+INSTANTIATE_TEST_SUITE_P(SystemStateTest, SystemStateReadOnlyTest, testing::Values(true, false));

tests/unit/etl/WriterStateTests.cpp

@@ -0,0 +1,162 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of clio: https://github.com/XRPLF/clio
+    Copyright (c) 2025, the clio developers.
+
+    Permission to use, copy, modify, and distribute this software for any
+    purpose with or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include "etl/SystemState.hpp"
+#include "etl/WriterState.hpp"
+#include "util/MockPrometheus.hpp"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include <memory>
+
+using namespace etl;
+using namespace testing;
+
+struct WriterStateTest : util::prometheus::WithPrometheus {
+    std::shared_ptr<SystemState> systemState = std::make_shared<SystemState>();
+    StrictMock<MockFunction<void(SystemState::WriteCommand)>> mockWriteCommand;
+    WriterState writerState{systemState};
+
+    WriterStateTest()
+    {
+        systemState->writeCommandSignal.connect(mockWriteCommand.AsStdFunction());
+    }
+};
+
+TEST_F(WriterStateTest, IsWritingReturnsSystemStateValue)
+{
+    systemState->isWriting = false;
+    EXPECT_FALSE(writerState.isWriting());
+
+    systemState->isWriting = true;
+    EXPECT_TRUE(writerState.isWriting());
+}
+
+TEST_F(WriterStateTest, StartWritingEmitsStartWritingCommand)
+{
+    systemState->isWriting = false;
+
+    EXPECT_CALL(mockWriteCommand, Call(SystemState::WriteCommand::StartWriting));
+
+    writerState.startWriting();
+}
+
+TEST_F(WriterStateTest, StartWritingDoesNothingWhenAlreadyWriting)
+{
+    systemState->isWriting = true;
+
+    // No EXPECT_CALL - StrictMock will fail if any command is emitted
+
+    writerState.startWriting();
+}
+
+TEST_F(WriterStateTest, GiveUpWritingEmitsStopWritingCommand)
+{
+    systemState->isWriting = true;
+
+    EXPECT_CALL(mockWriteCommand, Call(SystemState::WriteCommand::StopWriting));
+
+    writerState.giveUpWriting();
+}
+
+TEST_F(WriterStateTest, GiveUpWritingDoesNothingWhenNotWriting)
+{
+    systemState->isWriting = false;
+
+    // No EXPECT_CALL - StrictMock will fail if any command is emitted
+
+    writerState.giveUpWriting();
+}
+
+TEST_F(WriterStateTest, IsFallbackReturnsFalseByDefault)
+{
+    EXPECT_FALSE(writerState.isFallback());
+}
+
+TEST_F(WriterStateTest, SetWriterDecidingFallbackSetsFlag)
+{
+    EXPECT_FALSE(systemState->isWriterDecidingFallback);
+
+    writerState.setWriterDecidingFallback();
+
+    EXPECT_TRUE(systemState->isWriterDecidingFallback);
+}
+
+TEST_F(WriterStateTest, IsFallbackReturnsSystemStateValue)
+{
+    systemState->isWriterDecidingFallback = false;
+    EXPECT_FALSE(writerState.isFallback());
+
+    systemState->isWriterDecidingFallback = true;
+    EXPECT_TRUE(writerState.isFallback());
+}
+
+TEST_F(WriterStateTest, IsReadOnlyReturnsSystemStateValue)
+{
+    systemState->isStrictReadonly = false;
+    EXPECT_FALSE(writerState.isReadOnly());
+
+    systemState->isStrictReadonly = true;
+    EXPECT_TRUE(writerState.isReadOnly());
+}
+
+TEST_F(WriterStateTest, IsLoadingCacheReturnsSystemStateValue)
+{
+    systemState->isLoadingCache = false;
+    EXPECT_FALSE(writerState.isLoadingCache());
+
+    systemState->isLoadingCache = true;
+    EXPECT_TRUE(writerState.isLoadingCache());
+}
+
+TEST_F(WriterStateTest, CloneCreatesNewInstanceWithSameSystemState)
+{
+    systemState->isWriting = true;
+    systemState->isStrictReadonly = true;
+    systemState->isLoadingCache = false;
+
+    auto cloned = writerState.clone();
+
+    ASSERT_NE(cloned.get(), &writerState);
+    EXPECT_TRUE(cloned->isWriting());
+    EXPECT_TRUE(cloned->isReadOnly());
+    EXPECT_FALSE(cloned->isLoadingCache());
+}
+
+TEST_F(WriterStateTest, ClonedInstanceSharesSystemState)
+{
+    auto cloned = writerState.clone();
+
+    systemState->isWriting = true;
+
+    EXPECT_TRUE(writerState.isWriting());
+    EXPECT_TRUE(cloned->isWriting());
+
+    systemState->isWriting = false;
+
+    EXPECT_FALSE(writerState.isWriting());
+    EXPECT_FALSE(cloned->isWriting());
+
+    EXPECT_FALSE(writerState.isFallback());
+    EXPECT_FALSE(cloned->isFallback());
+    cloned->setWriterDecidingFallback();
+    EXPECT_TRUE(writerState.isFallback());
+    EXPECT_TRUE(cloned->isFallback());
+}

tests/unit/etlng/LedgerPublisherTests.cpp

@@ -1,356 +0,0 @@
-//------------------------------------------------------------------------------
-/*
-    This file is part of clio: https://github.com/XRPLF/clio
-    Copyright (c) 2025, the clio developers.
-
-    Permission to use, copy, modify, and distribute this software for any
-    purpose with or without fee is hereby granted, provided that the above
-    copyright notice and this permission notice appear in all copies.
-
-    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
-    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
-    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
-    ANY  SPECIAL,  DIRECT,  INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
-    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
-    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
-*/
-//==============================================================================
-
-#include "data/DBHelpers.hpp"
-#include "data/Types.hpp"
-#include "etl/SystemState.hpp"
-#include "util/AsioContextTestFixture.hpp"
-#include "util/MockBackendTestFixture.hpp"
-#include "util/MockPrometheus.hpp"
-#include "util/MockSubscriptionManager.hpp"
-#include "util/TestObject.hpp"
-#include "util/config/ConfigDefinition.hpp"
-
-#include <etlng/impl/LedgerPublisher.hpp>
-#include <fmt/format.h>
-#include <gmock/gmock.h>
-#include <gtest/gtest.h>
-#include <xrpl/basics/chrono.h>
-#include <xrpl/protocol/Indexes.h>
-#include <xrpl/protocol/LedgerHeader.h>
-
-#include <chrono>
-#include <optional>
-#include <vector>
-
-using namespace testing;
-using namespace etlng;
-using namespace data;
-using namespace std::chrono;
-
-namespace {
-
-constexpr auto kACCOUNT = "rf1BiGeXwwQoi8Z2ueFYTEXSwuJYfV2Jpn";
-constexpr auto kACCOUNT2 = "rLEsXccBGNR3UPuPu2hUXPjziKC3qKSBun";
-constexpr auto kLEDGER_HASH = "4BC50C9B0D8515D3EAAE1E74B29A95804346C491EE1A95BF25E4AAB854A6A652";
-constexpr auto kSEQ = 30;
-constexpr auto kAGE = 800;
-constexpr auto kAMOUNT = 100;
-constexpr auto kFEE = 3;
-constexpr auto kFINAL_BALANCE = 110;
-constexpr auto kFINAL_BALANCE2 = 30;
-
-MATCHER_P(ledgerHeaderMatcher, expectedHeader, "Headers match")
-{
-    return arg.seq == expectedHeader.seq && arg.hash == expectedHeader.hash &&
-        arg.closeTime == expectedHeader.closeTime;
-}
-
-}  // namespace
-
-struct ETLLedgerPublisherNgTest : util::prometheus::WithPrometheus, MockBackendTestStrict, SyncAsioContextTest {
-    util::config::ClioConfigDefinition cfg{{}};
-    StrictMockSubscriptionManagerSharedPtr mockSubscriptionManagerPtr;
-};
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerHeaderSkipDueToAge)
-{
-    // Use kAGE (800) which is > MAX_LEDGER_AGE_SECONDS (600) to test skipping
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, kAGE);
-    auto dummyState = etl::SystemState{};
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-
-    backend_->setRange(kSEQ - 1, kSEQ);
-    publisher.publish(dummyLedgerHeader);
-
-    // Verify last published sequence is set immediately
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    // Since age > MAX_LEDGER_AGE_SECONDS, these should not be called
-    EXPECT_CALL(*backend_, doFetchLedgerObject).Times(0);
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger).Times(0);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger).Times(0);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges).Times(0);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction).Times(0);
-
-    ctx_.run();
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerHeaderWithinAgeLimit)
-{
-    // Use age 0 which is < MAX_LEDGER_AGE_SECONDS to ensure publishing happens
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, 0);
-    auto dummyState = etl::SystemState{};
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-
-    backend_->setRange(kSEQ - 1, kSEQ);
-    publisher.publish(dummyLedgerHeader);
-
-    // Verify last published sequence is set immediately
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger(kSEQ, _))
-        .WillOnce(Return(std::vector<TransactionAndMetadata>{}));
-
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(_, _, fmt::format("{}-{}", kSEQ - 1, kSEQ), 0));
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges);
-
-    ctx_.run();
-    EXPECT_TRUE(publisher.lastPublishAgeSeconds() <= 1);
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerHeaderIsWritingTrue)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, kAGE);
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    publisher.publish(dummyLedgerHeader);
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    ctx_.run();
-    EXPECT_FALSE(backend_->fetchLedgerRange());
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerHeaderInRange)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, 0);  // age is 0
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    backend_->setRange(kSEQ - 1, kSEQ);
-
-    publisher.publish(dummyLedgerHeader);
-
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-
-    TransactionAndMetadata t1;
-    t1.transaction =
-        createPaymentTransactionObject(kACCOUNT, kACCOUNT2, kAMOUNT, kFEE, kSEQ).getSerializer().peekData();
-    t1.metadata = createPaymentTransactionMetaObject(kACCOUNT, kACCOUNT2, kFINAL_BALANCE, kFINAL_BALANCE2)
-                      .getSerializer()
-                      .peekData();
-    t1.ledgerSequence = kSEQ;
-
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger).WillOnce(Return(std::vector<TransactionAndMetadata>{t1}));
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(_, _, fmt::format("{}-{}", kSEQ - 1, kSEQ), 1));
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges);
-    // mock 1 transaction
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction);
-
-    ctx_.run();
-    EXPECT_TRUE(publisher.lastPublishAgeSeconds() <= 1);
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerHeaderCloseTimeGreaterThanNow)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-
-    auto dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, 0);
-    auto const nowPlus10 = system_clock::now() + seconds(10);
-    auto const closeTime = duration_cast<seconds>(nowPlus10.time_since_epoch()).count() - kRIPPLE_EPOCH_START;
-    dummyLedgerHeader.closeTime = ripple::NetClock::time_point{seconds{closeTime}};
-
-    backend_->setRange(kSEQ - 1, kSEQ);
-
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    publisher.publish(dummyLedgerHeader);
-
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-
-    TransactionAndMetadata t1;
-    t1.transaction =
-        createPaymentTransactionObject(kACCOUNT, kACCOUNT2, kAMOUNT, kFEE, kSEQ).getSerializer().peekData();
-    t1.metadata = createPaymentTransactionMetaObject(kACCOUNT, kACCOUNT2, kFINAL_BALANCE, kFINAL_BALANCE2)
-                      .getSerializer()
-                      .peekData();
-    t1.ledgerSequence = kSEQ;
-
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger(kSEQ, _))
-        .WillOnce(Return(std::vector<TransactionAndMetadata>{t1}));
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(_, _, fmt::format("{}-{}", kSEQ - 1, kSEQ), 1));
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction);
-
-    ctx_.run();
-    EXPECT_TRUE(publisher.lastPublishAgeSeconds() <= 1);
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerSeqStopIsTrue)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isStopping = true;
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    EXPECT_FALSE(publisher.publish(kSEQ, {}));
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerSeqMaxAttempt)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isStopping = false;
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-
-    static constexpr auto kMAX_ATTEMPT = 2;
-
-    LedgerRange const range{.minSequence = kSEQ - 1, .maxSequence = kSEQ - 1};
-    EXPECT_CALL(*backend_, hardFetchLedgerRange).Times(kMAX_ATTEMPT).WillRepeatedly(Return(range));
-
-    EXPECT_FALSE(publisher.publish(kSEQ, kMAX_ATTEMPT, std::chrono::milliseconds{1}));
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishLedgerSeqStopIsFalse)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isStopping = false;
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-
-    LedgerRange const range{.minSequence = kSEQ, .maxSequence = kSEQ};
-    EXPECT_CALL(*backend_, hardFetchLedgerRange).WillOnce(Return(range));
-
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, kAGE);
-    EXPECT_CALL(*backend_, fetchLedgerBySequence(kSEQ, _)).WillOnce(Return(dummyLedgerHeader));
-
-    EXPECT_TRUE(publisher.publish(kSEQ, {}));
-    ctx_.run();
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishMultipleTxInOrder)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, 0);  // age is 0
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    backend_->setRange(kSEQ - 1, kSEQ);
-
-    publisher.publish(dummyLedgerHeader);
-
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-
-    // t1 index > t2 index
-    TransactionAndMetadata t1;
-    t1.transaction =
-        createPaymentTransactionObject(kACCOUNT, kACCOUNT2, kAMOUNT, kFEE, kSEQ).getSerializer().peekData();
-    t1.metadata = createPaymentTransactionMetaObject(kACCOUNT, kACCOUNT2, kFINAL_BALANCE, kFINAL_BALANCE2, 2)
-                      .getSerializer()
-                      .peekData();
-    t1.ledgerSequence = kSEQ;
-    t1.date = 1;
-    TransactionAndMetadata t2;
-    t2.transaction =
-        createPaymentTransactionObject(kACCOUNT, kACCOUNT2, kAMOUNT, kFEE, kSEQ).getSerializer().peekData();
-    t2.metadata = createPaymentTransactionMetaObject(kACCOUNT, kACCOUNT2, kFINAL_BALANCE, kFINAL_BALANCE2, 1)
-                      .getSerializer()
-                      .peekData();
-    t2.ledgerSequence = kSEQ;
-    t2.date = 2;
-
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger(kSEQ, _))
-        .WillOnce(Return(std::vector<TransactionAndMetadata>{t1, t2}));
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(_, _, fmt::format("{}-{}", kSEQ - 1, kSEQ), 2));
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges);
-
-    Sequence const s;
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction(t2, _)).InSequence(s);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction(t1, _)).InSequence(s);
-
-    ctx_.run();
-    EXPECT_TRUE(publisher.lastPublishAgeSeconds() <= 1);
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishVeryOldLedgerShouldSkip)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-
-    // Create a ledger header with age (800) greater than MAX_LEDGER_AGE_SECONDS (600)
-    auto const dummyLedgerHeader = createLedgerHeader(kLEDGER_HASH, kSEQ, 800);
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    backend_->setRange(kSEQ - 1, kSEQ);
-
-    publisher.publish(dummyLedgerHeader);
-
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger).Times(0);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges).Times(0);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubTransaction).Times(0);
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ);
-
-    ctx_.run();
-}
-
-TEST_F(ETLLedgerPublisherNgTest, PublishMultipleLedgersInQuickSuccession)
-{
-    auto dummyState = etl::SystemState{};
-    dummyState.isWriting = true;
-
-    auto const dummyLedgerHeader1 = createLedgerHeader(kLEDGER_HASH, kSEQ, 0);
-    auto const dummyLedgerHeader2 = createLedgerHeader(kLEDGER_HASH, kSEQ + 1, 0);
-    auto publisher = etlng::impl::LedgerPublisher(ctx_, backend_, mockSubscriptionManagerPtr, dummyState);
-    backend_->setRange(kSEQ - 1, kSEQ + 1);
-
-    // Publish two ledgers in quick succession
-    publisher.publish(dummyLedgerHeader1);
-    publisher.publish(dummyLedgerHeader2);
-
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-    EXPECT_CALL(*backend_, doFetchLedgerObject(ripple::keylet::fees().key, kSEQ + 1, _))
-        .WillOnce(Return(createLegacyFeeSettingBlob(1, 2, 3, 4, 0)));
-
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger(kSEQ, _))
-        .WillOnce(Return(std::vector<TransactionAndMetadata>{}));
-    EXPECT_CALL(*backend_, fetchAllTransactionsInLedger(kSEQ + 1, _))
-        .WillOnce(Return(std::vector<TransactionAndMetadata>{}));
-
-    Sequence const s;
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(ledgerHeaderMatcher(dummyLedgerHeader1), _, _, _)).InSequence(s);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges(ledgerHeaderMatcher(dummyLedgerHeader1), _)).InSequence(s);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubLedger(ledgerHeaderMatcher(dummyLedgerHeader2), _, _, _)).InSequence(s);
-    EXPECT_CALL(*mockSubscriptionManagerPtr, pubBookChanges(ledgerHeaderMatcher(dummyLedgerHeader2), _)).InSequence(s);
-
-    EXPECT_TRUE(publisher.getLastPublishedSequence());
-    EXPECT_EQ(publisher.getLastPublishedSequence().value(), kSEQ + 1);
-
-    ctx_.run();
-}

tests/unit/util/ChannelTests.cpp

@@ -187,7 +187,7 @@ TEST_P(ChannelSpawnTest, MultipleSendersMultipleReceivers)
     context_.withExecutor([this](auto& executor) {
         auto [sender, receiver] = util::Channel<int>::create(executor, 10);
         util::Mutex<std::vector<int>> receivedValues;
-        std::vector<decltype(receiver)> receivers(kNUM_RECEIVERS, receiver);
+        std::vector receivers(kNUM_RECEIVERS, receiver);
 
         for (auto receiverId = 0uz; receiverId < kNUM_RECEIVERS; ++receiverId) {
             util::spawn(
@@ -402,7 +402,7 @@ TEST_P(ChannelCallbackTest, MultipleSendersMultipleReceivers)
     context_.withExecutor([this](auto& executor) {
         auto [sender, receiver] = util::Channel<int>::create(executor, 10);
         util::Mutex<std::vector<int>> receivedValues;
-        std::vector<decltype(receiver)> receivers(kNUM_RECEIVERS, receiver);
+        std::vector receivers(kNUM_RECEIVERS, receiver);
 
         for (auto receiverId = 0uz; receiverId < kNUM_RECEIVERS; ++receiverId) {
             auto& receiverRef = receivers[receiverId];
@@ -528,8 +528,8 @@ TEST_P(ChannelCallbackTest, TryMethodsWithClosedChannel)
     context_.withExecutor([this](auto& executor) {
         std::atomic_bool testCompleted{false};
         auto [sender, receiver] = util::Channel<int>::create(executor, 3);
-        auto receiverPtr = std::make_shared<decltype(receiver)>(std::move(receiver));
-        auto senderPtr = std::make_shared<std::optional<decltype(sender)>>(std::move(sender));
+        auto receiverPtr = std::make_shared<util::Channel<int>::Receiver>(std::move(receiver));
+        auto senderPtr = std::make_shared<std::optional<util::Channel<int>::Sender>>(std::move(sender));
 
         boost::asio::post(executor, [receiverPtr, senderPtr, &testCompleted]() {
             EXPECT_TRUE(senderPtr->value().trySend(100));