rippled/docs/build/telemetry.md

OpenTelemetry Tracing for xrpld

This document explains how to build xrpld with OpenTelemetry distributed tracing support, configure the runtime telemetry options, and set up the observability backend to view traces.

• OpenTelemetry Tracing for xrpld
  • Overview
  • Building with Telemetry
    • Summary
    • Build steps
      • Install dependencies
      • Call CMake
      • Build
  • Building without telemetry
  • Troubleshooting
    • Conan lockfile error
    • CMake target not found
  • Conditional compilation
  • Recording utilities
  • Span lifetime and cross-thread handling
    • SpanGuard versus ScopedSpanGuard
    • Coroutine-aware context storage
    • Handing a span to a job
    • Why are unrelated spans in my trace?
    • Injecting trace context into a protobuf message

Overview

xrpld supports optional OpenTelemetry distributed tracing. When enabled, it instruments RPC requests with trace spans that are exported via OTLP/HTTP to an OpenTelemetry Collector, which forwards them to a tracing backend such as Grafana Tempo.

Telemetry is gated twice — once at compile time and once at runtime:

• Compile time: The Conan option telemetry must be True. It is the only switch: there is no CMake option, because conan install writes the value into the generated toolchain and CMake reads it from there. When off, all SpanGuard calls compile to inline no-ops (defined in SpanGuard.h) with zero overhead — no OTel SDK dependency required. Pass the value you want explicitly rather than relying on the default.
• Runtime: Telemetry is off by default — the [telemetry] config section must set enabled=1. When disabled at runtime, a no-op implementation is used even in a build that has the SDK compiled in.

Building with Telemetry

Summary

Follow the same instructions as mentioned in BUILD.md but with the following changes:

1. Pass -o telemetry=True to conan install to pull the opentelemetry-cpp dependency.
2. CMake will automatically pick up telemetry=ON from the Conan-generated toolchain.
3. Build as usual.

---

Build steps

cd /path/to/xrpld
rm -rf .build
mkdir .build
cd .build

Install dependencies

The telemetry option adds opentelemetry-cpp/1.28.0 as a dependency. If the Conan lockfile does not yet include this package, bypass it with --lockfile="".

conan install .. \
    --output-folder . \
    --build missing \
    --settings build_type=Debug \
    -o telemetry=True \
    -o tests=True \
    -o xrpld=True \
    --lockfile=""

Note

: The first build with telemetry may take longer as opentelemetry-cpp and its transitive dependencies are compiled from source.

Call CMake

The Conan-generated toolchain file sets telemetry=ON automatically. No additional CMake flags are needed beyond the standard ones.

cmake .. -G Ninja \
    -DCMAKE_TOOLCHAIN_FILE:FILEPATH=build/generators/conan_toolchain.cmake \
    -DCMAKE_BUILD_TYPE=Debug \
    -Dtests=ON -Dxrpld=ON

You should see in the CMake output:

-- OpenTelemetry tracing enabled

Build

cmake --build . --parallel $(nproc)

Building without telemetry

Pass -o telemetry=False to conan install. That is the whole switch: CMake takes the value from the generated toolchain, so there is no CMake flag to pass as well. Do not just omit the option — it then resolves to whatever the recipe's current default is.

The opentelemetry-cpp dependency will not be downloaded, the XRPL_ENABLE_TELEMETRY preprocessor define will not be set, and all tracing macros will compile to no-ops. The resulting binary is identical to one built before telemetry support was added.

-DXRPL_ENABLE_TELEMETRY=OFF disables nothing. XRPL_ENABLE_TELEMETRY is not a CMake option — it is only a compile definition added when telemetry is on. Passing it on the command line leaves telemetry compiled in; CMake merely lists it at the end of configuration under Manually-specified variables were not used by the project. Use -o telemetry=False on conan install.

Troubleshooting

Conan lockfile error

If you see ERROR: Requirement 'opentelemetry-cpp/1.28.0' not in lockfile 'requires', the lockfile was generated without the telemetry dependency. Pass --lockfile="" to bypass the lockfile, or regenerate it with telemetry enabled.

CMake target not found

If CMake reports that opentelemetry-cpp targets are not found, ensure you ran conan install with -o telemetry=True and that the Conan-generated toolchain file is being used. The Conan package provides a single umbrella target opentelemetry-cpp::opentelemetry-cpp (not individual component targets).

Conditional compilation

All OpenTelemetry SDK types are hidden behind the pimpl idiom in SpanGuard.cpp. When XRPL_ENABLE_TELEMETRY is not defined, SpanGuard.h provides an all-inline no-op stub class with no OTel dependencies. At runtime, if enabled=0 is set in config (or the section is omitted), a NullTelemetry implementation is used that returns no-op spans.

Those two layers remove the span, but they do not remove the work that computes what you pass to it. The compiled-out guards are ordinary inline functions with ordinary parameters, so every argument is evaluated before the empty body is entered:

// to_string() allocates a 64-character string even in a build with telemetry
// compiled out. The call then does nothing with it.
span.setAttribute(attr::txHash, to_string(txID).c_str());

Guard the work, not just the call. Testing the guard is enough: its operator bool() is a literal false when telemetry is compiled out, so the whole block is eliminated, and when telemetry is compiled in it also skips the work if tracing is switched off in config or the span's category is disabled.

if (span)
    span.setAttribute(attr::txHash, to_string(txID).c_str());

A span that exists but was sampled out still pays: there is no isRecording() to test.

The XRPL_METRIC_* macros are the opposite case. They expand to do { } while (false) and discard their arguments, so anything named only inside a macro argument list disappears on its own and needs no guard.

Recording utilities

Some state exists only to be reported: a timestamp read to measure something, a counter nothing outside telemetry reads, a value kept so that a change in it can be logged. Writing that with preprocessor branches puts #ifdef through business logic and leaves the class with a different member set in each build — a difference that has previously made a test mock abstract.

xrpl/telemetry/Recording.h holds that state in types that carry a real member when telemetry is compiled in and are empty types with no-op methods when it is not. Declare the member unconditionally: its storage collapses to padding, and its work disappears.

Utility	Use it for	With telemetry compiled out
kEnabled	if constexpr (telemetry::kEnabled) around a telemetry-only block that has no span to test	false, so the block is discarded
Stopwatch	an elapsed time measured only in order to report it	holds nothing; elapsedUs() returns exactly 0
Counter<T>	a count with no reader outside telemetry	holds nothing; load() returns T{}

// Times a loop with no preprocessor branch anywhere. The clock is not read at
// all in a build with telemetry compiled out.
telemetry::Stopwatch const timer;
for (auto const& obj : objects)
    lookUp(obj);
recordLookupMetrics(timer.elapsedUs());

Two constraints decide whether these are usable at a given site:

• A no-op method does not skip its arguments. counter.add(expensiveCount()) still calls expensiveCount(). Pass values that are cheap to produce, and put anything expensive inside if constexpr (telemetry::kEnabled).
• if constexpr still type-checks the branch it discards in non-template code, so use it only where the block names no opentelemetry:: type. SpanGuard exists to keep those types out of call sites, so that is the usual case; a block that does name them stays behind #ifdef.

Counter declares copy and move deleted, matching the std::atomic it holds when telemetry is compiled in, so a class that owns one has the same copy semantics in both builds.

Span lifetime and cross-thread handling

Telemetry exposes two RAII guards with split responsibilities, plus a non-owning activation helper. Picking the right one is what keeps a trace's parent/child nesting and its per-line log correlation correct.

SpanGuard versus ScopedSpanGuard

• SpanGuard owns a span and nothing else. It is thread-free: it never touches the active-context stack, so it carries no thread affinity and may be moved to and ended on any thread. It is movable and move-assignable. Create one with SpanGuard::span(cat, prefix, name), or with SpanGuard::freshRoot(...) to start a fresh trace root that ignores whatever span is currently ambient. Reach for a plain SpanGuard whenever the span must leave the context store that created it — for example when it is handed into a job.

• ScopedSpanGuard owns a SpanGuard plus an active OTel scope that pushes the span onto the current context store. While it lives, the span is the ambient parent for child spans created on that store, and log lines emitted under it carry its trace_id. It is non-copyable and non-movable — short-lived stack RAII. It offers the same factories (freshRoot(...), childSpan(...)). When the span must outlive the scope, convert it with operator SpanGuard() &&: that pops the scope on the origin store and yields the bare, thread-free SpanGuard.

Rule of thumb: use ScopedSpanGuard for same-thread (or same-coroutine) nesting and log correlation; use the plain SpanGuard whenever the span crosses out of the store that created it.

flowchart TD
    SG["SpanGuard<br/>(unscoped, thread-free)<br/>owns span only; movable across threads and coroutines"]
    SSG["ScopedSpanGuard<br/>(scoped, store-bound)<br/>owns a SpanGuard plus an active scope on the current store"]
    SA["ScopedActivation<br/>(non-owning)<br/>activates a borrowed span; never owns or ends it"]

    SSG -->|"handoff: pop scope, yield bare span"| SG
    SG -->|"activate / activateIfLive"| SA
    SA -.->|"borrows span, no ownership"| SG

    classDef box fill:#e8f0fe,stroke:#3b5bdb,color:#111827;
    class SG,SSG,SA box;

Coroutine-aware context storage

The active-context stack is not a plain thread_local. At telemetry start xrpld installs CoroAwareContextStorage, which keeps the stack in an xrpl::LocalValue. Because JobQueue::Coro::resume() swaps the coroutine's LocalValue store in and out with the coroutine, the ambient context follows the coroutine across every yield and resume — even when it resumes on a different worker thread. A ScopedSpanGuard held across a coroutine yield is therefore safe: its scope rides the coroutine and pops on the same store it was pushed onto, so it never pops the wrong stack. Off a coroutine the LocalValue transparently gives each thread its own store, so behaviour matches OTel's default thread-local storage. This is what lets the RPC entry, process, and command spans be scoped — for correct nesting and per-line log-trace correlation — even though the RPC path yields.

Handing a span to a job

The hand-off pattern is: create a thread-free SpanGuard at the origin (or convert a ScopedSpanGuard with operator SpanGuard() &&), move it into the job closure, and inside the worker body activate it non-destructively with telemetry::activateIfLive(handle). That call takes no ownership and returns a ScopedActivation (a no-op if the handle is empty or the span inactive) which makes the span the ambient context so log lines in the worker body carry its trace_id. The activation neither owns nor ends the span — the owning SpanGuard still controls its lifetime and ends it when the closure is destroyed. Keep the activation confined to a synchronous, non-yielding block. There is no detach step: a SpanGuard is already thread-free.

Why are unrelated spans in my trace?

Historically a scoped guard destroyed off its origin thread popped the wrong context stack, leaving a stale ambient span in place that later work inherited. The current design removes that failure mode in two ways:

• Coroutine-aware storage makes a scope held across a coroutine yield pop on the same store it was pushed onto, so a coroutine that resumes on another worker never pops the wrong stack.

• A same-store assertion in ScopedSpanGuard (and ScopedActivation) records the LocalValue store its scope was pushed onto and checks, in debug/test/fuzzing builds, that destruction, hand-off, and discard all happen while that same store is active — turning a genuine cross-store misuse into an immediate assertion failure rather than a silently corrupted trace.

If a trace still shows unrelated spans nested under one operation, the usual cause is an inbound entry point that inherited an ambient parent it should not have. Start such an operation with freshRoot() so it begins a clean trace root and never adopts whatever span happened to be active. To move a span across a store boundary, keep it in a thread-free SpanGuard (or convert via operator SpanGuard() &&) rather than holding a ScopedSpanGuard across the boundary.

Injecting trace context into a protobuf message

Pass the whole message to the injection helpers, never *msg.mutable_trace_context().

On a protobuf optional submessage, mutable_ allocates the submessage and sets its has-bit, and that happens at the call site before the helper runs. A caller that dereferences it therefore puts an empty TraceContext on the wire whenever nothing is recorded, and every receiving peer takes its has_trace_context() branch to extract nothing from it. trace_context is field 1001, so the wasted bytes are a 2-byte tag plus a zero length.

// Right: the helper decides whether the submessage is created at all.
telemetry::injectSpanContext(span, msg);

// Wrong: the submessage exists before the helper can decide anything.
telemetry::injectSpanContext(span, *msg.mutable_trace_context());

injectCurrentContext(msg) does the same for whichever span is active on the calling thread, deciding via SpanGuard::hasCurrentContext(). Four states have to come out right:

Build	Runtime	Result
compiled out	n/a	no submessage; the bytes on the wire match a build without telemetry
compiled in	a span is active	trace_id, span_id and the trace flags are written
compiled in	no active span	no submessage, rather than an empty one
compiled in	enabled=0	no submessage

hasCurrentContext() reads the span straight out of the runtime context. opentelemetry::trace::GetSpan() would be shorter, but it returns a heap-allocated DefaultSpan when the context holds no span — an allocation in exactly the case the predicate exists to keep free.