created_at2022-09-07 13:47:46.52515
updated_at2023-08-15 18:46:10.726595
descriptionLibrary for common telemetry and observability functionality
Evan Chan (velvia)




Common utilities for Tokio-based application telemetry, including tracing, logging, spans

This is a library for common telemetry functionality, especially subscribers for Tokio tracing libraries. Here we simply package many common subscribers, such as writing trace data to Jaeger, distributed tracing, common logs and metrics destinations, etc. into a easy to configure common package. There are also some unique layers such as one to automatically create Prometheus latency histograms for spans.

We also purposely separate out logging levels from span creation. This is often needed by production apps as normally it is not desired to log at very high levels, but still desirable to gather sampled span data all the way down to TRACE level spans.

Getting started is easy. In your app:

  use telemetry_subscribers::TelemetryConfig;
  let (_guard, _handle) = TelemetryConfig::new("my_app").init();

It is important to retain the guard until the end of the program. Assign it in the main fn and keep it, for once it drops then log output will stop.

There is a builder API available: just do TelemetryConfig::new()... Another convenient initialization method is TelemetryConfig::new().with_env() to populate the config from environment vars.

You can also run the example and see output in ANSI color:

cargo run --example easy-init


  • jaeger - this feature is enabled by default as it enables jaeger tracing
  • json - Bunyan formatter - JSON log output, optional
  • tokio-console - Tokio-console subscriber, optional
  • chrome - enables use of chrome://tracing to visualize output, optional

Stdout vs file output

By default, logs (but not spans) are formatted for human readability and output to stdout, with key-value tags at the end of every line. RUST_LOG can be configured for custom logging output, including filtering.

By setting log_file in the config, one can write log output to a daily-rotated file.

Tracing and span output

Detailed span start and end logs can be generated in two ways:

  • by defining the span_log_output config variable / ENABLE_SPAN_LOGS env var.
  • by defining the json_log_output config variable / ENABLE_JSON_LOGS env var. Note that this causes output to be in JSON format, which is not as human-readable, so it is not enabled by default.

JSON output can easily be fed to backends such as ElasticSearch for indexing, alerts, aggregation, and ysis. It requires the json crate feature to be enabled.

Jaeger (seeing distributed traces)

To see nested spans visualized with Jaeger, do the following:

  1. Run this to get a local Jaeger container: docker run -d -p6831:6831/udp -p6832:6832/udp -p16686:16686 jaegertracing/all-in-one:latest
  2. Set enable_jaeger config setting to true or set TOKIO_JAEGER env var
  3. Run your app
  4. Browse to http://localhost:16686/ and select the service you configured using service_name

NOTE: separate spans (which are not nested) are not connected as a single trace for now.

Jaeger subscriber is enabled by default but is protected by the jaeger feature flag. If you'd like to leave out the Jaeger dependencies, you can turn off the default-features in your dependency:

telemetry = { url = "...", default-features = false }

Automatic Prometheus span latencies

Included in this library is a tracing-subscriber layer named PrometheusSpanLatencyLayer. It will create a Prometheus histogram to track latencies for every span in your app, which is super convenient for tracking span performance in production apps.

Enabling this layer can only be done programmatically, by passing in a Prometheus registry to TelemetryConfig.

Span levels vs log levels

What spans are included for Jaeger output, automatic span latencies, etc.? These are controlled by the span_level config attribute, or the TOKIO_SPAN_LEVEL environment variable. Note that this is separate from RUST_LOG, so that you can separately control the logging verbosity from the level of spans that are to be recorded and traced.

Note that span levels for regular logging output are not affected by the span level config.

Live async inspection / Tokio Console

Tokio-console is an awesome CLI tool designed to analyze and help debug Rust apps using Tokio, in real time! It relies on a special subscriber.

  1. Build your app using a special flag: RUSTFLAGS="--cfg tokio_unstable" cargo build
  2. Enable the tokio-console feature for this crate.
  3. Set the tokio_console config setting when running your app (or set TOKIO_CONSOLE env var if using config with_env() method)
  4. Clone the console repo and cargo run to launch the console

NOTE: setting tokio TRACE logs is NOT necessary. It says that in the docs but there's no need to change Tokio logging levels at all. The console subscriber has a special filter enabled taking care of that.

By default, Tokio console listens on port 6669. To change this setting as well as other setting such as the retention policy, please see the configuration guide.

Custom panic hook

This library installs a custom panic hook which records a log (event) at ERROR level using the tracing crate. This allows span information from the panic to be properly recorded as well.

To exit the process on panic, set the CRASH_ON_PANIC environment variable.

Commit count: 4

cargo fmt