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feat(appender-tracing): stabilize span attribute propagation (#3482)
1 parent f290595 commit cc87dd9

6 files changed

Lines changed: 310 additions & 102 deletions

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‎opentelemetry-appender-tracing/CHANGELOG.md‎

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@@ -2,10 +2,31 @@
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## vNext
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- New *experimental* feature to enrich log records with attributes from active
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tracing spans (`experimental_span_attributes`). Use
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`OpenTelemetryTracingBridge::builder()` with `with_span_attribute_allowlist`
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to control which span attributes are copied to log records.
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- **Add tracing span attribute enrichment.** When enabled, attributes attached
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to active [`tracing`] spans are copied onto each emitted log record. "Span"
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here refers to a [`tracing::span!`][tracing-span] from the [`tracing`] crate
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(the appender's source), **not** an OpenTelemetry span.
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Enrichment is **disabled by default** (no per-span overhead) and must be
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opted into at runtime via a single builder method that accepts a
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[`TracingSpanAttributes`] value:
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```rust
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use opentelemetry_appender_tracing::layer::TracingSpanAttributes;
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// Copy all tracing-span attributes onto log records:
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let layer = OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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// Or copy only an allowlist of attributes:
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let layer = OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::allowlist(["session.id"]))
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.build();
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```
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[`tracing`]: https://crates.io/crates/tracing
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[tracing-span]: https://docs.rs/tracing/latest/tracing/macro.span.html
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- Remove the `experimental_use_tracing_span_context` since
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`tracing-opentelemetry` now supports [activating][31901] the OpenTelemetry

‎opentelemetry-appender-tracing/Cargo.toml‎

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@@ -34,7 +34,6 @@ pprof = { workspace = true }
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[features]
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default = []
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experimental_metadata_attributes = ["dep:tracing-log"]
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experimental_span_attributes = []
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bench_profiling = []
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[[bench]]

‎opentelemetry-appender-tracing/benches/logs.rs‎

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@@ -14,13 +14,14 @@
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Hardware: Apple M4 Pro
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Total Number of Cores: 14 (10 performance and 4 efficiency)
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rustc 1.95.0 (59807616e 2026-04-14)
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| Test | Average time|
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|-----------------------------|-------------|
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| log_no_subscriber | 285 ps |
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| noop_layer_disabled | 8 ns |
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| noop_layer_enabled | 14 ns |
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| ot_layer_disabled | 12 ns |
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| ot_layer_enabled | 130 ns |
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| log_no_subscriber | 262 ps |
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| noop_layer_disabled | 4 ns |
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| noop_layer_enabled | 12 ns |
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| ot_layer_disabled | 7 ns |
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| ot_layer_enabled | 122 ns |
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*/
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use criterion::{criterion_group, criterion_main, Criterion};

‎opentelemetry-appender-tracing/benches/span-attributes.rs‎

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Original file line numberDiff line numberDiff line change
@@ -1,37 +1,39 @@
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/*
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// Run this benchmark with:
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// cargo bench --bench span-attributes --features experimental_span_attributes
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// cargo bench --bench span-attributes
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// The benchmark results:
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// Hardware: Mac M4 Pro
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// Total Number of Cores: 14 (10 performance and 4 efficiency)
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// rustc 1.93.0 (254b59607 2026-01-19)
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// cargo 1.93.0 (083ac5135 2025-12-15)
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// rustc 1.95.0 (59807616e 2026-04-14)
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// cargo 1.95.0 (f2d3ce0bd 2026-03-21)
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//
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// Attribute counts are chosen to stay within the SDK's inline-array optimisation
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// threshold (5 attributes). With `experimental_span_attributes` enabled, span
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// threshold (5 attributes). With tracing-span attribute enrichment enabled (via
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// `OpenTelemetryTracingBridgeBuilder::with_tracing_span_attributes`), tracing-span
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// attributes are copied onto the log record, so total = log attrs + span attrs.
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//
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// Log + span benchmarks (showing incremental cost of span attributes on logging):
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// | Test | Total attrs | Average time | Increment vs baseline |
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// |-------------------------------------------|-------------|--------------|------------------------------|
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// | log_1_attr_no_span | 1 | 81 ns | - |
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// | log_1_attr_in_span_2_attr | 3 | 325 ns | +244 ns |
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// | log_1_attr_in_nested_spans_2plus2_attr | 5 | 570 ns | +489 ns (+245 ns vs 1 span) |
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// | log_1_attr_no_span | 1 | 85 ns | - |
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// | log_1_attr_in_span_2_attr | 3 | 382 ns | +297 ns |
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// | log_1_attr_in_nested_spans_2plus2_attr | 5 | 590 ns | +505 ns (+208 ns vs 1 span) |
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//
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// Span-only benchmarks (no log emission, kept for reference):
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// | Test | Average time | Increment |
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// |-----------------------|--------------|-----------|
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// | span_4_attributes | 175 ns | - |
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// | span_8_attributes | 272 ns | +97 ns |
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// | nested_spans_1_levels | 183 ns | - |
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// | nested_spans_2_levels | 385 ns | +202 ns |
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// | nested_spans_3_levels | 583 ns | +198 ns |
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// | span_4_attributes | 194 ns | - |
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// | span_8_attributes | 314 ns | +120 ns |
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// | nested_spans_1_levels | 194 ns | - |
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// | nested_spans_2_levels | 413 ns | +219 ns |
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// | nested_spans_3_levels | 630 ns | +217 ns |
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*/
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use criterion::{criterion_group, criterion_main, Criterion};
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use opentelemetry::InstrumentationScope;
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use opentelemetry_appender_tracing::layer as tracing_layer;
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use opentelemetry_appender_tracing::layer::TracingSpanAttributes;
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use opentelemetry_sdk::error::OTelSdkResult;
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use opentelemetry_sdk::logs::{LogProcessor, SdkLogRecord, SdkLoggerProvider};
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use opentelemetry_sdk::Resource;
@@ -67,7 +69,9 @@ fn benchmark_span_attributes(c: &mut Criterion, num_attributes: usize) {
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.with_log_processor(NoopProcessor)
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.build();
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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let subscriber = Registry::default().with(ot_layer);
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tracing::subscriber::with_default(subscriber, || {
@@ -129,7 +133,9 @@ fn benchmark_nested_spans(c: &mut Criterion, depth: usize) {
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.with_log_processor(NoopProcessor)
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.build();
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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let subscriber = Registry::default().with(ot_layer);
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tracing::subscriber::with_default(subscriber, || {
@@ -246,7 +252,9 @@ fn make_provider() -> SdkLoggerProvider {
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/// 5-attribute inline-array optimisation threshold.
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fn benchmark_log_1_attr_no_span(c: &mut Criterion) {
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let provider = make_provider();
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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let subscriber = Registry::default().with(ot_layer);
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tracing::subscriber::with_default(subscriber, || {
@@ -264,11 +272,13 @@ fn benchmark_log_1_attr_no_span(c: &mut Criterion) {
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/// Emit an error log with 1 attribute while inside a span that carries 2 attributes
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/// (total = 3 attrs on the log record, within the SDK's 5-attr inline threshold).
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/// With `experimental_span_attributes` the 2 span attributes are propagated onto the
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/// With tracing-span attribute enrichment enabled, the 2 span attributes are copied onto the
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/// log record, so the delta vs `log_1_attr_no_span` shows the pure span-attribute cost.
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fn benchmark_log_1_attr_in_span_2_attr(c: &mut Criterion) {
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let provider = make_provider();
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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let subscriber = Registry::default().with(ot_layer);
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tracing::subscriber::with_default(subscriber, || {
@@ -293,7 +303,9 @@ fn benchmark_log_1_attr_in_span_2_attr(c: &mut Criterion) {
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/// two spans) vs the same total number of span attributes on a single span.
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fn benchmark_log_1_attr_in_nested_spans_2plus2_attr(c: &mut Criterion) {
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let provider = make_provider();
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::new(&provider);
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let ot_layer = tracing_layer::OpenTelemetryTracingBridge::builder(&provider)
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.with_tracing_span_attributes(TracingSpanAttributes::all())
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.build();
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let subscriber = Registry::default().with(ot_layer);
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tracing::subscriber::with_default(subscriber, || {

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