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opentelemetry-appender-tracing

Version: 0.31.1 The opentelemetry-appender-tracing crate bridges tracing events to OpenTelemetry logs, allowing applications using the tracing ecosystem to export structured logs via OpenTelemetry.
This is different from tracing-opentelemetry, which converts tracing spans into OpenTelemetry traces. This crate specifically handles tracing events as OpenTelemetry logs.

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Installation

[dependencies]
tracing = ">=0.1.40"
tracing-subscriber = { version = "0.3", features = ["registry", "std"] }
opentelemetry = { version = "0.31", features = ["logs"] }
opentelemetry-sdk = { version = "0.31", features = ["logs"] }
opentelemetry-appender-tracing = "0.31"

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Feature Flags

  • experimental_metadata_attributes: Include code metadata (file path, line number, module) as attributes
  • experimental_span_attributes: Include parent span information as attributes

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Quick Start

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Basic Setup

use opentelemetry_sdk::logs::SdkLoggerProvider;
use opentelemetry_stdout::LogExporter;
use opentelemetry_appender_tracing::layer::OpenTelemetryTracingBridge;
use tracing_subscriber::prelude::*;

// 1. Set up OpenTelemetry logger provider
let exporter = LogExporter::default();
let provider = SdkLoggerProvider::builder()
    .with_simple_exporter(exporter)
    .build();

// 2. Create the OpenTelemetry-tracing bridge layer
let otel_layer = OpenTelemetryTracingBridge::new(&provider);

// 3. Register with tracing subscriber
tracing_subscriber::registry()
    .with(otel_layer)
    .with(tracing_subscriber::fmt::layer()) // Optional: also log to stdout
    .init();

// 4. Use tracing as normal
tracing::info!("Application started");
tracing::error!("Something went wrong");

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Structured Events

use tracing::error;

error!(
    name: "my-event-name",
    target: "my-system",
    event_id = 10,
    user_name = "alice",
    user_email = "[email protected]",
    message = "This is an example message"
);

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Core Types

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layer::OpenTelemetryTracingBridge
struct
A tracing_subscriber::Layer that forwards tracing events to OpenTelemetry logs
use opentelemetry_appender_tracing::layer::OpenTelemetryTracingBridge;
use opentelemetry_sdk::logs::SdkLoggerProvider;

let provider: SdkLoggerProvider = /* ... */;
let layer = OpenTelemetryTracingBridge::new(&provider);

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Field Mapping

How tracing events map to OpenTelemetry logs:

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Event Name

tracing events can optionally have a name, which becomes the OpenTelemetry EventName: 
tracing::info!(name: "user.login", user_id = 42);
// -> LogRecord.EventName = "user.login"

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Target

The target field maps to OpenTelemetry InstrumentationScope: 
tracing::info!(target: "http_server", "Request received");
// -> InstrumentationScope.name = "http_server"

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Severity

tracing::Level maps to OpenTelemetry severity:
tracing::LevelSeverity TextSeverity Number
ERRORError17
WARNWarn13
INFOInfo9
DEBUGDebug5
TRACETrace1

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Fields → Attributes

tracing event fields become OpenTelemetry attributes, except:
  • Field named "message" → LogRecord::Body
  • If no "message" field exists, the event’s formatted message → LogRecord::Body
tracing::info!(
    user_id = 42,
    action = "login",
    message = "User logged in"
);
// -> Body = "User logged in"
// -> Attributes: { user_id: 42, action: "login" }

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Type Conversion

tracing TypeOpenTelemetry TypeNotes
i64AnyValue::Int
u64, u128, i128AnyValue::Int or StringConverted if fits in i64, else stringified
f32, f64AnyValue::Double
boolAnyValue::Boolean
&strAnyValue::String
&[u8]AnyValue::BytesBinary data
&dyn DebugAnyValue::StringVia Debug formatting
&dyn ErrorAnyValue::StringStored in exception.message attribute

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Automatic Context Attachment

The bridge automatically attaches OpenTelemetry trace context to logs: 
use tracing::info;
use opentelemetry::trace::{Tracer, TraceContextExt};
use opentelemetry::Context;

let tracer = /* get tracer */;
tracer.in_span("my_operation", |cx| {
    info!("Processing item");
    // Log automatically includes:
    // - trace_id
    // - span_id
    // - trace_flags
});

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Examples

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Basic Logging

use tracing::{info, warn, error};

info!("Server started on port 8080");
warn!("High memory usage detected");
error!("Database connection failed");

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Named Events

use tracing::error;

error!(
    name: "database.connection.failed",
    database = "users_db",
    retry_count = 3,
    "Failed to connect after retries"
);

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With Explicit Message

use tracing::info;

info!(
    user_id = 42,
    action = "purchase",
    amount = 99.99,
    message = "User completed purchase"
);

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Exception Logging

use tracing::error;
use std::error::Error;

fn handle_error(err: &dyn Error) {
    error!(
        error = err as &dyn Error,
        "Operation failed"
    );
}

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With Parent Spans

use tracing::{info, info_span};

let span = info_span!("request", request_id = "123");
let _guard = span.enter();

info!("Processing request");
// Log includes parent span context automatically

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Integration with OTLP

use opentelemetry_otlp::LogExporter;
use opentelemetry_sdk::logs::{BatchLogProcessor, SdkLoggerProvider};
use opentelemetry_appender_tracing::layer::OpenTelemetryTracingBridge;
use tracing_subscriber::prelude::*;

let exporter = LogExporter::builder()
    .with_http()
    .build()?;

let provider = SdkLoggerProvider::builder()
    .with_log_processor(BatchLogProcessor::builder(exporter).build())
    .build();

let otel_layer = OpenTelemetryTracingBridge::new(&provider);

tracing_subscriber::registry()
    .with(otel_layer)
    .init();

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Combining with Tracing Spans

For converting tracing spans to OpenTelemetry traces, use the third-party tracing-opentelemetry crate. Both can be used together:
  • tracing-opentelemetry: Spans → OpenTelemetry Traces
  • opentelemetry-appender-tracing: Events → OpenTelemetry Logs
use tracing_subscriber::prelude::*;

// Hypothetical combined setup
tracing_subscriber::registry()
    .with(tracing_opentelemetry::layer()) // Spans → Traces
    .with(OpenTelemetryTracingBridge::new(&logger_provider)) // Events → Logs
    .with(tracing_subscriber::fmt::layer()) // Also print to console
    .init();

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Metadata Attributes (Experimental)

With experimental_metadata_attributes feature: 
[dependencies]
opentelemetry-appender-tracing = { version = "0.31", features = ["experimental_metadata_attributes"] }
Automatically includes:
  • code.filepath: Source file path
  • code.line_number: Line number
  • code.function_name: Module path

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Performance Considerations

  • Events are only processed if the level is enabled
  • Use BatchLogProcessor for better throughput
  • Field serialization has overhead; avoid excessive fields
  • The bridge integrates as a Layer, allowing composition with other layers

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Differences from tracing-opentelemetry

Featureopentelemetry-appender-tracingtracing-opentelemetry
PurposeEvents → OpenTelemetry LogsSpans → OpenTelemetry Traces
Maintained byOpenTelemetry projectThird-party (Tokio)
Event handlingConverts to logsConverts to span events
Span handlingNot supportedConverts to traces

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Documentation

Full API Documentation

View complete API reference on docs.rs

tracing Crate

Documentation for the tracing crate

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Minimum Rust Version

MSRV: 1.75.0

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