tracing

Application-level tracing for Rust.

Website | Chat | Documentation (master branch)

Overview

tracing is a framework for instrumenting Rust programs to collect structured, event-based diagnostic information. tracing is maintained by the Tokio project, but does not require the tokio runtime to be used.

Usage

(The examples below are borrowed from the log crate's yak-shaving example, modified to idiomatic tracing.)

In Applications

In order to record trace events, executables have to use a Subscriber implementation compatible with tracing. A Subscriber implements a way of collecting trace data, such as by logging it to standard output. tracing_subscriber's fmt module provides reasonable defaults. Additionally, tracing-subscriber is able to consume messages emitted by log-instrumented libraries and modules.

The simplest way to use a subscriber is to call the set_global_default function.

use tracing::{info, Level};
use tracing_subscriber::FmtSubscriber;

fn main() {
    // a builder for `FmtSubscriber`.
    let subscriber = FmtSubscriber::builder()
        // all spans/events with a level higher than TRACE (e.g, debug, info, warn, etc.)
        // will be written to stdout.
        .with_max_level(Level::TRACE)
        // completes the builder.
        .finish();

    tracing::subscriber::set_global_default(subscriber)
        .expect("setting defualt subscriber failed");

    let number_of_yaks = 3;
    // this creates a new event, outside of any spans.
    info!(number_of_yaks, "preparing to shave yaks");

    let number_shaved = yak_shave::shave_all(number_of_yaks);
    info!(
        all_yaks_shaved = number_shaved == number_of_yaks,
        "yak shaving completed."
    );
}

[dependencies]
tracing = "0.1"
tracing-subscriber = "0.2.0-alpha.5"

This subscriber will be used as the default in all threads for the remainder of the duration of the program, similar to how loggers work in the log crate.

In addition, you can locally override the default subscriber. For example:

use tracing::{info, Level};
use tracing_subscruber::FmtSubscriber;

fn main() {
    let subscriber = tracing_subscriber::FmtSubscriber::builder()
        // all spans/events with a level higher than TRACE (e.g, debug, info, warn, etc.)
        // will be written to stdout.
        .with_max_level(Level::TRACE)
        // builds the subscriber.
        .finish();

    tracing::subscriber::with_default(subscriber, || {
        info!("This will be logged to stdout");
    });
    info!("This will _not_ be logged to stdout");
}

Any trace events generated outside the context of a subscriber will not be collected.

This approach allows trace data to be collected by multiple subscribers within different contexts in the program. Note that the override only applies to the currently executing thread; other threads will not see the change from with_default.

Once a subscriber has been set, instrumentation points may be added to the executable using the tracing crate's macros.

In Libraries

Libraries should only rely on the tracing crate and use the provided macros and types to collect whatever information might be useful to downstream consumers.

use std::{error::Error, io};
use tracing::{debug, error, info, span, warn, Level};

// the `#[tracing::instrument]` attribute creates and enters a span
// every time the instrumented function is called. The span is named after the
// the function or method. Paramaters passed to the function are recorded as fields.
#[tracing::instrument]
pub fn shave(yak: usize) -> Result<(), Box<dyn Error + 'static>> {
    // this creates an event at the DEBUG level with two fields:
    // - `excitement`, with the key "excitement" and the value "yay!"
    // - `message`, with the key "message" and the value "hello! I'm gonna shave a yak."
    //
    // unlike other fields, `message`'s shorthand initialization is just the string itself.
    debug!(excitement = "yay!", "hello! I'm gonna shave a yak.");
    if yak == 3 {
        warn!("could not locate yak!");
        // note that this is intended to demonstrate `tracing`'s features, not idiomatic
        // error handling! in a library or application, you should consider returning
        // a dedicated `YakError`. libraries like snafu or thiserror make this easy.
        return Err(io::Error::new(io::ErrorKind::Other, "shaving yak failed!").into());
    } else {
        debug!("yak shaved successfully");
    }
    Ok(())
}

pub fn shave_all(yaks: usize) -> usize {
    // Constructs a new span named "shaving_yaks" at the TRACE level,
    // and a field whose key is "yaks". This is equivalent to writing:
    //
    // let span = span!(Level::TRACE, "shaving_yaks", yaks = yaks);
    //
    // local variables (`yaks`) can be used as field values
    // without an assignment, similar to struct initializers.
    let span = span!(Level::TRACE, "shaving_yaks", yaks);
    let _enter = span.enter();

    info!("shaving yaks");

    let mut yaks_shaved = 0;
    for yak in 1..=yaks {
        let res = shave(yak);
        debug!(yak, shaved = res.is_ok());

        if let Err(ref error) = res {
            // Like spans, events can also use the field initialization shorthand.
            // In this instance, `yak` is the field being initalized.
            error!(yak, error = error.as_ref(), "failed to shave yak!");
        } else {
            yaks_shaved += 1;
        }
        debug!(yaks_shaved);
    }

    yaks_shaved
}

[dependencies]
tracing = "0.1"

Note: Libraries should NOT call set_global_default(), as this will cause conflicts when executables try to set the default later.

In Asynchronous Code

If you are instrumenting code that make use of std::future::Future or async/await, be sure to use the tracing-futures crate. This is needed because the following example will not work:

async {
    let _s = span.enter();
    // ...
}

The span guard _s will not exit until the future generated by the async block is complete. Since futures and spans can be entered and exited multiple times without them completing, the span remains entered for as long as the future exists, rather than being entered only when it is polled, leading to very confusing and incorrect output. For more details, see the documentation on closing spans.

There are two ways to instrument asynchronous code. The first is through the Future::instrument combinator:

use tracing_futures::Instrument;

let my_future = async {
    // ...
};

my_future
    .instrument(tracing::info_span!("my_future"))
    .await

Future::instrument attaches a span to the future, ensuring that the span's lifetime is as long as the future's.

The second, and preferred, option is through the #[instrument] attribute:

use tracing::{info, instrument};
use tokio::{io::AsyncWriteExt, net::TcpStream};
use std::io;

#[instrument]
async fn write(stream: &mut TcpStream) -> io::Result<usize> {
    let result = stream.write(b"hello world\n").await;
    info!("wrote to stream; success={:?}", result.is_ok());
    result
}

Under the hood, the #[instrument] macro performs same the explicit span attachment that Future::instrument does.

Note: the #[tracing::instrument]` macro does not work correctly with the async-trait crate. This bug is tracked in #399.

Getting Help

First, see if the answer to your question can be found in the API documentation. If the answer is not there, there is an active community in the Tracing Discord channel. We would be happy to try to answer your question. Last, if that doesn't work, try opening an issue with the question.

Contributing

🎈 Thanks for your help improving the project! We are so happy to have you! We have a contributing guide to help you get involved in the Tracing project.

Project layout

The tracing crate contains the primary instrumentation API, used for instrumenting libraries and applications to emit trace data. The tracing-core crate contains the core API primitives on which the rest of tracing is instrumented. Authors of trace subscribers may depend on tracing-core, which guarantees a higher level of stability.

Additionally, this repository contains several compatibility and utility libraries built on top of tracing. Some of these crates are in a pre-release state, and are less stable than the tracing and tracing-core crates.

The crates included as part of Tracing are:

• tracing-futures: Utilities for instrumenting futures. (crates.io|docs)

• tracing-macros: Experimental macros for emitting trace events (unstable).

• tracing-attributes: Procedural macro attributes for automatically instrumenting functions. (crates.io|docs)

• tracing-log: Compatibility with the log crate (unstable).

• tracing-serde: A compatibility layer for serializing trace data with serde (unstable).

• tracing-subscriber: Subscriber implementations, and utilities for implementing and composing Subscribers. (crates.io|docs)

• tracing-tower: Compatibility with the tower ecosystem (unstable).

Related Crates

In addition to this repository, here are also several third-party crates which are not maintained by the tokio project. These include:

• tracing-timing implements inter-event timing metrics on top of tracing. It provides a subscriber that records the time elapsed between pairs of tracing events and generates histograms.
• tracing-opentelemetry provides a subscriber for emitting traces to OpenTelemetry-compatible distributed tracing systems.
• tracing-honeycomb implements a subscriber for reporting traces to honeycomb.io.
• tracing-actix provides tracing integration for the actix actor framework.
• tracing-gelf implements a subscriber for exporting traces in Greylog GELF format.
• tracing-coz provides integration with the coz causal profiler (Linux-only).

(if you're the maintainer of a tracing ecosystem crate not in this list, please let us know!)

Note: that some of the ecosystem crates are currently unreleased and undergoing active development. They may be less stable than tracing and tracing-core.

External Resources

This is a list of links to blog posts, conference talks, and tutorials about Tracing.

Blog Posts

• Diagnostics with Tracing on the Tokio blog, August 2019

Talks

• Bay Area Rust Meetup talk and Q&A, March 2018
• RustConf 2019 talk and slides, August 2019

Help us expand this list! If you've written or spoken about Tracing, or know of resources that aren't listed, please open a pull request adding them.

License

This project is licensed under the MIT license.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tracing by you, shall be licensed as MIT, without any additional terms or conditions.