feat: ETLng task manager (#1843)

src/etlng/CMakeLists.txt

@@ -2,7 +2,7 @@ add_library(clio_etlng)
 
 target_sources(
   clio_etlng PRIVATE impl/AmendmentBlockHandler.cpp impl/AsyncGrpcCall.cpp impl/Extraction.cpp impl/GrpcSource.cpp
-                     impl/Loading.cpp
+                     impl/Loading.cpp impl/TaskManager.cpp
 )
 
 target_link_libraries(clio_etlng PUBLIC clio_data)

src/etlng/LedgerPublisherInterface.hpp

@@ -0,0 +1,49 @@
+//------------------------------------------------------------------------------
+/*
+    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 <chrono>
+#include <cstdint>
+#include <optional>
+
+namespace etlng {
+
+/**
+ * @brief The interface of a scheduler for the extraction proccess
+ */
+struct LedgerPublisherInterface {
+    virtual ~LedgerPublisherInterface() = default;
+
+    /**
+     * @brief Publish the ledger by its sequence number
+     *
+     * @param seq The sequence number of the ledger
+     * @param maxAttempts The maximum number of attempts to publish the ledger; no limit if nullopt
+     * @param attemptsDelay The delay between attempts
+     */
+    virtual void
+    publish(
+        uint32_t seq,
+        std::optional<uint32_t> maxAttempts,
+        std::chrono::steady_clock::duration attemptsDelay = std::chrono::seconds{1}
+    ) = 0;
+};
+
+}  // namespace etlng

src/etlng/impl/TaskManager.cpp

@@ -0,0 +1,143 @@
+//------------------------------------------------------------------------------
+/*
+    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 "etlng/impl/TaskManager.hpp"
+
+#include "etlng/ExtractorInterface.hpp"
+#include "etlng/LoaderInterface.hpp"
+#include "etlng/Models.hpp"
+#include "etlng/SchedulerInterface.hpp"
+#include "util/async/AnyExecutionContext.hpp"
+#include "util/async/AnyOperation.hpp"
+#include "util/async/AnyStrand.hpp"
+#include "util/log/Logger.hpp"
+
+#include <xrpl/protocol/TxFormats.h>
+
+#include <chrono>
+#include <cstddef>
+#include <functional>
+#include <ranges>
+#include <thread>
+#include <utility>
+#include <vector>
+
+namespace etlng::impl {
+
+TaskManager::TaskManager(
+    util::async::AnyExecutionContext&& ctx,
+    std::reference_wrapper<SchedulerInterface> scheduler,
+    std::reference_wrapper<ExtractorInterface> extractor,
+    std::reference_wrapper<LoaderInterface> loader
+)
+    : ctx_(std::move(ctx)), schedulers_(scheduler), extractor_(extractor), loader_(loader)
+{
+}
+
+TaskManager::~TaskManager()
+{
+    stop();
+}
+
+void
+TaskManager::run(Settings settings)
+{
+    static constexpr auto kQUEUE_SIZE_LIMIT = 2048uz;
+
+    auto schedulingStrand = ctx_.makeStrand();
+    PriorityQueue queue(ctx_.makeStrand(), kQUEUE_SIZE_LIMIT);
+
+    LOG(log_.debug()) << "Starting task manager...\n";
+
+    extractors_.reserve(settings.numExtractors);
+    for ([[maybe_unused]] auto _ : std::views::iota(0uz, settings.numExtractors))
+        extractors_.push_back(spawnExtractor(schedulingStrand, queue));
+
+    loaders_.reserve(settings.numLoaders);
+    for ([[maybe_unused]] auto _ : std::views::iota(0uz, settings.numLoaders))
+        loaders_.push_back(spawnLoader(queue));
+
+    wait();
+    LOG(log_.debug()) << "All finished in task manager..\n";
+}
+
+util::async::AnyOperation<void>
+TaskManager::spawnExtractor(util::async::AnyStrand& strand, PriorityQueue& queue)
+{
+    // TODO: these values may be extracted to config later and/or need to be fine-tuned on a realistic system
+    static constexpr auto kDELAY_BETWEEN_ATTEMPTS = std::chrono::milliseconds{100u};
+    static constexpr auto kDELAY_BETWEEN_ENQUEUE_ATTEMPTS = std::chrono::milliseconds{1u};
+
+    return strand.execute([this, &queue](auto stopRequested) {
+        while (not stopRequested) {
+            if (auto task = schedulers_.get().next(); task.has_value()) {
+                if (auto maybeBatch = extractor_.get().extractLedgerWithDiff(task->seq); maybeBatch.has_value()) {
+                    LOG(log_.debug()) << "Adding data after extracting diff";
+                    while (not queue.enqueue(*maybeBatch)) {
+                        // TODO (https://github.com/XRPLF/clio/issues/1852)
+                        std::this_thread::sleep_for(kDELAY_BETWEEN_ENQUEUE_ATTEMPTS);
+
+                        if (stopRequested)
+                            break;
+                    }
+                } else {
+                    // TODO: how do we signal to the loaders that it's time to shutdown? some special task?
+                    break;  // TODO: handle server shutdown or other node took over ETL
+                }
+            } else {
+                // TODO (https://github.com/XRPLF/clio/issues/1852)
+                std::this_thread::sleep_for(kDELAY_BETWEEN_ATTEMPTS);
+            }
+        }
+    });
+}
+
+util::async::AnyOperation<void>
+TaskManager::spawnLoader(PriorityQueue& queue)
+{
+    return ctx_.execute([this, &queue](auto stopRequested) {
+        while (not stopRequested) {
+            // TODO (https://github.com/XRPLF/clio/issues/66): does not tell the loader whether it's out of order or not
+            if (auto data = queue.dequeue(); data.has_value())
+                loader_.get().load(*data);
+        }
+    });
+}
+
+void
+TaskManager::wait()
+{
+    for (auto& extractor : extractors_)
+        extractor.wait();
+    for (auto& loader : loaders_)
+        loader.wait();
+}
+
+void
+TaskManager::stop()
+{
+    for (auto& extractor : extractors_)
+        extractor.abort();
+    for (auto& loader : loaders_)
+        loader.abort();
+
+    wait();
+}
+
+}  // namespace etlng::impl

src/etlng/impl/TaskManager.hpp

@@ -0,0 +1,94 @@
+//------------------------------------------------------------------------------
+/*
+    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 "etlng/ExtractorInterface.hpp"
+#include "etlng/LoaderInterface.hpp"
+#include "etlng/Models.hpp"
+#include "etlng/SchedulerInterface.hpp"
+#include "util/StrandedPriorityQueue.hpp"
+#include "util/async/AnyExecutionContext.hpp"
+#include "util/async/AnyOperation.hpp"
+#include "util/async/AnyStrand.hpp"
+#include "util/log/Logger.hpp"
+
+#include <xrpl/protocol/TxFormats.h>
+
+#include <cstddef>
+#include <functional>
+#include <vector>
+
+namespace etlng::impl {
+
+class TaskManager {
+    util::async::AnyExecutionContext ctx_;
+    std::reference_wrapper<SchedulerInterface> schedulers_;
+    std::reference_wrapper<ExtractorInterface> extractor_;
+    std::reference_wrapper<LoaderInterface> loader_;
+
+    std::vector<util::async::AnyOperation<void>> extractors_;
+    std::vector<util::async::AnyOperation<void>> loaders_;
+
+    util::Logger log_{"ETL"};
+
+    struct ReverseOrderComparator {
+        [[nodiscard]] bool
+        operator()(model::LedgerData const& lhs, model::LedgerData const& rhs) const noexcept
+        {
+            return lhs.seq > rhs.seq;
+        }
+    };
+
+public:
+    struct Settings {
+        size_t numExtractors; /**< number of extraction tasks */
+        size_t numLoaders;    /**< number of loading tasks */
+    };
+
+    // reverse order loading is needed (i.e. start with oldest seq in forward fill buffer)
+    using PriorityQueue = util::StrandedPriorityQueue<model::LedgerData, ReverseOrderComparator>;
+
+    TaskManager(
+        util::async::AnyExecutionContext&& ctx,
+        std::reference_wrapper<SchedulerInterface> scheduler,
+        std::reference_wrapper<ExtractorInterface> extractor,
+        std::reference_wrapper<LoaderInterface> loader
+    );
+
+    ~TaskManager();
+
+    void
+    run(Settings settings);
+
+    void
+    stop();
+
+private:
+    void
+    wait();
+
+    [[nodiscard]] util::async::AnyOperation<void>
+    spawnExtractor(util::async::AnyStrand& strand, PriorityQueue& queue);
+
+    [[nodiscard]] util::async::AnyOperation<void>
+    spawnLoader(PriorityQueue& queue);
+};
+
+}  // namespace etlng::impl

src/feed/SubscriptionManager.cpp

@@ -50,7 +50,7 @@ void
 SubscriptionManager::pubBookChanges(
     ripple::LedgerHeader const& lgrInfo,
     std::vector<data::TransactionAndMetadata> const& transactions
-) const
+)
 {
     bookChangesFeed_.pub(lgrInfo, transactions);
 }
@@ -111,7 +111,7 @@ SubscriptionManager::pubLedger(
     ripple::Fees const& fees,
     std::string const& ledgerRange,
     std::uint32_t const txnCount
-) const
+)
 {
     ledgerFeed_.pub(lgrInfo, fees, ledgerRange, txnCount);
 }
@@ -129,7 +129,7 @@ SubscriptionManager::unsubManifest(SubscriberSharedPtr const& subscriber)
 }
 
 void
-SubscriptionManager::forwardManifest(boost::json::object const& manifestJson) const
+SubscriptionManager::forwardManifest(boost::json::object const& manifestJson)
 {
     manifestFeed_.pub(manifestJson);
 }
@@ -147,7 +147,7 @@ SubscriptionManager::unsubValidation(SubscriberSharedPtr const& subscriber)
 }
 
 void
-SubscriptionManager::forwardValidation(boost::json::object const& validationJson) const
+SubscriptionManager::forwardValidation(boost::json::object const& validationJson)
 {
     validationsFeed_.pub(validationJson);
 }

src/feed/SubscriptionManager.hpp

@@ -150,7 +150,7 @@ public:
      */
     void
     pubBookChanges(ripple::LedgerHeader const& lgrInfo, std::vector<data::TransactionAndMetadata> const& transactions)
-        const final;
+        final;
 
     /**
      * @brief Subscribe to the proposed transactions feed.
@@ -218,7 +218,7 @@ public:
         ripple::Fees const& fees,
         std::string const& ledgerRange,
         std::uint32_t txnCount
-    ) const final;
+    ) final;
 
     /**
      * @brief Subscribe to the manifest feed.
@@ -239,7 +239,7 @@ public:
      * @param manifestJson The manifest json to forward.
      */
     void
-    forwardManifest(boost::json::object const& manifestJson) const final;
+    forwardManifest(boost::json::object const& manifestJson) final;
 
     /**
      * @brief Subscribe to the validation feed.
@@ -260,7 +260,7 @@ public:
      * @param validationJson The validation feed json to forward.
      */
     void
-    forwardValidation(boost::json::object const& validationJson) const final;
+    forwardValidation(boost::json::object const& validationJson) final;
 
     /**
      * @brief Subscribe to the transactions feed.

src/feed/SubscriptionManagerInterface.hpp

@@ -71,8 +71,10 @@ public:
      * @param transactions The transactions in the current ledger.
      */
     virtual void
-    pubBookChanges(ripple::LedgerHeader const& lgrInfo, std::vector<data::TransactionAndMetadata> const& transactions)
-        const = 0;
+    pubBookChanges(
+        ripple::LedgerHeader const& lgrInfo,
+        std::vector<data::TransactionAndMetadata> const& transactions
+    ) = 0;
 
     /**
      * @brief Subscribe to the proposed transactions feed.
@@ -141,7 +143,7 @@ public:
         ripple::Fees const& fees,
         std::string const& ledgerRange,
         std::uint32_t txnCount
-    ) const = 0;
+    ) = 0;
 
     /**
      * @brief Subscribe to the manifest feed.
@@ -162,7 +164,7 @@ public:
      * @param manifestJson The manifest json to forward.
      */
     virtual void
-    forwardManifest(boost::json::object const& manifestJson) const = 0;
+    forwardManifest(boost::json::object const& manifestJson) = 0;
 
     /**
      * @brief Subscribe to the validation feed.
@@ -183,7 +185,7 @@ public:
      * @param validationJson The validation feed json to forward.
      */
     virtual void
-    forwardValidation(boost::json::object const& validationJson) const = 0;
+    forwardValidation(boost::json::object const& validationJson) = 0;
 
     /**
      * @brief Subscribe to the transactions feed.

src/feed/impl/BookChangesFeed.hpp

@@ -48,7 +48,7 @@ struct BookChangesFeed : public SingleFeedBase {
      * @param transactions The transactions that were included in the ledger.
      */
     void
-    pub(ripple::LedgerHeader const& lgrInfo, std::vector<data::TransactionAndMetadata> const& transactions) const
+    pub(ripple::LedgerHeader const& lgrInfo, std::vector<data::TransactionAndMetadata> const& transactions)
     {
         SingleFeedBase::pub(boost::json::serialize(rpc::computeBookChanges(lgrInfo, transactions)));
     }

src/feed/impl/ForwardFeed.hpp

@@ -36,7 +36,7 @@ struct ForwardFeed : public SingleFeedBase {
      * @brief Publishes the json object.
      */
     void
-    pub(boost::json::object const& json) const
+    pub(boost::json::object const& json)
     {
         SingleFeedBase::pub(boost::json::serialize(json));
     }

src/feed/impl/LedgerFeed.cpp

@@ -94,7 +94,7 @@ LedgerFeed::pub(
     ripple::Fees const& fees,
     std::string const& ledgerRange,
     std::uint32_t const txnCount
-) const
+)
 {
     SingleFeedBase::pub(boost::json::serialize(makeLedgerPubMessage(lgrInfo, fees, ledgerRange, txnCount)));
 }

src/feed/impl/LedgerFeed.hpp

@@ -76,7 +76,7 @@ public:
     pub(ripple::LedgerHeader const& lgrInfo,
         ripple::Fees const& fees,
         std::string const& ledgerRange,
-        std::uint32_t txnCount) const;
+        std::uint32_t txnCount);
 
 private:
     static boost::json::object

src/feed/impl/SingleFeedBase.cpp

@@ -62,7 +62,7 @@ SingleFeedBase::unsub(SubscriberSharedPtr const& subscriber)
 }
 
 void
-SingleFeedBase::pub(std::string msg) const
+SingleFeedBase::pub(std::string msg)
 {
     [[maybe_unused]] auto task = strand_.execute([this, msg = std::move(msg)]() {
         auto const msgPtr = std::make_shared<std::string>(msg);

src/feed/impl/SingleFeedBase.hpp

@@ -73,7 +73,7 @@ public:
      * @param msg The message.
      */
     void
-    pub(std::string msg) const;
+    pub(std::string msg);
 
     /**
      * @brief Get the count of subscribers.

src/util/StrandedPriorityQueue.hpp

@@ -0,0 +1,116 @@
+//------------------------------------------------------------------------------
+/*
+    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/async/AnyStrand.hpp"
+
+#include <cstddef>
+#include <functional>
+#include <optional>
+#include <queue>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+namespace util {
+
+/**
+ * @brief A wrapper for std::priority_queue that serialises operations using a strand
+ * @note This may be a candidate for future improvements if performance proves to be poor (e.g. use a lock free queue)
+ */
+template <typename T, typename Compare = std::less<T>>
+class StrandedPriorityQueue {
+    util::async::AnyStrand strand_;
+    std::size_t limit_;
+    std::priority_queue<T, std::vector<T>, Compare> queue_;
+
+public:
+    /**
+     * @brief Construct a new priority queue on a strand
+     * @param strand The strand to use
+     * @param limit The limit of items allowed simultaniously in the queue
+     */
+    StrandedPriorityQueue(util::async::AnyStrand&& strand, std::optional<std::size_t> limit = std::nullopt)
+        : strand_(std::move(strand)), limit_(limit.value_or(0uz))
+    {
+    }
+
+    /**
+     * @brief Enqueue a new item onto the queue if space is available
+     * @note This function blocks until the item is attempted to be added to the queue
+     *
+     * @tparam I Type of the item to add
+     * @param item The item to add
+     * @return true if item added to the queue; false otherwise
+     */
+    template <typename I>
+    [[nodiscard]] bool
+    enqueue(I&& item)
+        requires std::is_same_v<std::decay_t<I>, T>
+    {
+        return strand_
+            .execute([&item, this] {
+                if (limit_ == 0uz or queue_.size() < limit_) {
+                    queue_.push(std::forward<I>(item));
+                    return true;
+                }
+                return false;
+            })
+            .get()
+            .value_or(false);  // if some exception happens - failed to add
+    }
+
+    /**
+     * @brief Dequeue the next available item out of the queue
+     * @note This function blocks until the item is taken off the queue
+     * @return An item if available; nullopt otherwise
+     */
+    [[nodiscard]] std::optional<T>
+    dequeue()
+    {
+        return strand_
+            .execute([this] -> std::optional<T> {
+                std::optional<T> out;
+
+                if (not queue_.empty()) {
+                    out.emplace(queue_.top());
+                    queue_.pop();
+                }
+
+                return out;
+            })
+            .get()
+            .value_or(std::nullopt);
+    }
+
+    /**
+     * @brief Check if the queue is empty
+     * @note This function blocks until the queue is checked
+     *
+     * @return true if the queue is empty; false otherwise
+     */
+    [[nodiscard]] bool
+    empty()
+    {
+        return strand_.execute([this] { return queue_.empty(); }).get().value();
+    }
+};
+
+}  // namespace util

src/util/async/AnyStrand.hpp

@@ -19,6 +19,7 @@
 
 #pragma once
 
+#include "util/async/AnyOperation.hpp"
 #include "util/async/AnyStopToken.hpp"
 #include "util/async/Concepts.hpp"
 #include "util/async/impl/ErasedOperation.hpp"
@@ -61,7 +62,7 @@ public:
      * @return The type-erased operation
      */
     [[nodiscard]] auto
-    execute(SomeHandlerWithoutStopToken auto&& fn) const
+    execute(SomeHandlerWithoutStopToken auto&& fn)
     {
         using RetType = std::decay_t<decltype(fn())>;
         static_assert(not std::is_same_v<RetType, std::any>);
@@ -85,7 +86,7 @@ public:
      * @return The type-erased operation
      */
     [[nodiscard]] auto
-    execute(SomeHandlerWith<AnyStopToken> auto&& fn) const
+    execute(SomeHandlerWith<AnyStopToken> auto&& fn)
     {
         using RetType = std::decay_t<decltype(fn(std::declval<AnyStopToken>()))>;
         static_assert(not std::is_same_v<RetType, std::any>);
@@ -110,7 +111,7 @@ public:
      * @return The type-erased operation
      */
     [[nodiscard]] auto
-    execute(SomeHandlerWith<AnyStopToken> auto&& fn, SomeStdDuration auto timeout) const
+    execute(SomeHandlerWith<AnyStopToken> auto&& fn, SomeStdDuration auto timeout)
     {
         using RetType = std::decay_t<decltype(fn(std::declval<AnyStopToken>()))>;
         static_assert(not std::is_same_v<RetType, std::any>);
@@ -137,7 +138,7 @@ private:
         [[nodiscard]] virtual impl::ErasedOperation
         execute(std::function<std::any(AnyStopToken)>, std::optional<std::chrono::milliseconds> timeout = std::nullopt)
             const = 0;
-        [[nodiscard]] virtual impl::ErasedOperation execute(std::function<std::any()>) const = 0;
+        [[nodiscard]] virtual impl::ErasedOperation execute(std::function<std::any()>) = 0;
     };
 
     template <typename StrandType>
@@ -158,7 +159,7 @@ private:
         }
 
         [[nodiscard]] impl::ErasedOperation
-        execute(std::function<std::any()> fn) const override
+        execute(std::function<std::any()> fn) override
         {
             return strand.execute(std::move(fn));
         }