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tracing

Application-level tracing for Rust.

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Overview

tracing is a framework for instrumenting Rust programs to collect structured, event-based diagnostic information.

In asynchronous systems like Tokio, interpreting traditional log messages can often be quite challenging. Since individual tasks are multiplexed on the same thread, associated events and log lines are intermixed making it difficult to trace the logic flow. tracing expands upon logging-style diagnostics by allowing libraries and applications to record structured events with additional information about temporality and causality — unlike a log message, a span in tracing has a beginning and end time, may be entered and exited by the flow of execution, and may exist within a nested tree of similar spans. In addition, tracing spans are structured, with the ability to record typed data as well as textual messages.

The tracing crate provides the APIs necessary for instrumenting libraries and applications to emit trace data.

Compiler support: requires rustc 1.39+

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 default 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"

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");
}

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.

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

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.

Concepts

This crate provides macros for creating Spans and Events, which represent periods of time and momentary events within the execution of a program, respectively.

As a rule of thumb, spans should be used to represent discrete units of work (e.g., a given request's lifetime in a server) or periods of time spent in a given context (e.g., time spent interacting with an instance of an external system, such as a database). In contrast, events should be used to represent points in time within a span — a request returned with a given status code, n new items were taken from a queue, and so on.

Spans are constructed using the span! macro, and then entered to indicate that some code takes place within the context of that Span:

use tracing::{span, Level};

// Construct a new span named "my span".
let mut span = span!(Level::INFO, "my span");
span.in_scope(|| {
    // Any trace events in this closure or code called by it will occur within
    // the span.
});
// Dropping the span will close it, indicating that it has ended.

The #[instrument] attribute macro can reduce some of this boilerplate:

use tracing::{instrument};

#[instrument]
pub fn my_function(my_arg: usize) {
    // This event will be recorded inside a span named `my_function` with the
    // field `my_arg`.
    tracing::info!("inside my_function!");
    // ...
}

The Event type represent an event that occurs instantaneously, and is essentially a Span that cannot be entered. They are created using the event! macro:

use tracing::{event, Level};

event!(Level::INFO, "something has happened!");

Users of the log crate should note that tracing exposes a set of macros for creating Events (trace!, debug!, info!, warn!, and error!) which may be invoked with the same syntax as the similarly-named macros from the log crate. Often, the process of converting a project to use tracing can begin with a simple drop-in replacement.

Ecosystem

Related Crates

In addition to tracing and tracing-core, the tokio-rs/tracing repository contains several additional crates designed to be used with the tracing ecosystem. This includes a collection of Subscriber implementations, as well as utility and adapter crates to assist in writing Subscribers and instrumenting applications.

In particular, the following crates are likely to be of interest:

  • tracing-futures provides a compatibility layer with the futures crate, allowing spans to be attached to Futures, Streams, and Executors.
  • tracing-subscriber provides Subscriber implementations and utilities for working with Subscribers. This includes a FmtSubscriber FmtSubscriber for logging formatted trace data to stdout, with similar filtering and formatting to the env_logger crate.
  • tracing-log provides a compatibility layer with the log crate, allowing log messages to be recorded as tracing Events within the trace tree. This is useful when a project using tracing have dependencies which use log. Note that if you're using tracing-subscriber's FmtSubscriber, you don't need to depend on tracing-log directly.

Additionally, there 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 Provides a layer that reports traces spanning multiple machines to honeycomb.io. Backed by tracing-distributed.
  • tracing-distributed Provides a generic implementation of a layer that reports traces spanning multiple machines to some backend.
  • 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 listed above, please let us know! We'd love to add your project to the list!

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

License

This project is licensed under the MIT license.

Contribution

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

Dependencies