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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.

Usage

First, add this to your Cargo.toml:

[dependencies]
tracing = "0.1.9"

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 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.

Let's consider the log crate's yak-shaving example, modified to use tracing:

// Import `tracing`'s macros rather than `log`'s
use tracing::{span, info, warn, Level};

// unchanged from here forward
pub fn shave_the_yak(yak: &mut Yak) {
    info!(target: "yak_events", "Commencing yak shaving for {:?}", yak);

    loop {
        match find_a_razor() {
            Ok(razor) => {
                info!("Razor located: {}", razor);
                yak.shave(razor);
                break;
            }
            Err(err) => {
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}

// Dummy impls to make the example compile
#[derive(Debug)] pub struct Yak(String);
impl Yak { fn shave(&mut self, _: u32) {} }
fn find_a_razor() -> Result<u32, u32> { Ok(1) }

We can change it even further to better utilize features in tracing.

use tracing::{span, info, warn, Level};

pub fn shave_the_yak(yak: &mut Yak) {
    // create and enter a span to represent the scope
    let span = span!(Level::TRACE, "shave_the_yak", ?yak);
    let _enter = span.enter();

    // Since the span is annotated with the yak, it is part of the context
    // for everything happening inside the span. Therefore, we don't need
    // to add it to the message for this event, as the `log` crate does.
    info!(target: "yak_events", "Commencing yak shaving");
    loop {
        match find_a_razor() {
            Ok(razor) => {
                // We can add the razor as a field rather than formatting it
                // as part of the message, allowing subscribers to consume it
                // in a more structured manner:
                info!(%razor, "Razor located");
                yak.shave(razor);
                break;
            }
            Err(err) => {
                // However, we can also create events with formatted messages,
                // just as we would for log records.
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}

#[derive(Debug)] pub struct Yak(String);
impl Yak { fn shave(&mut self, _: u32) {} }
fn find_a_razor() -> Result<u32, u32> { Ok(1) }

You can find further examples showing how to use this crate in the examples directory.

In libraries

Libraries should link only to the tracing crate, and use the provided macros to record whatever information will be useful to downstream consumers.

In executables

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.

There currently aren't too many subscribers to choose from. The best one to use right now is probably [tracing-fmt], which logs to the terminal. It is not currently published to crates.io so you will need to add [tracing-fmt] as a git dependency to use it.

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

use tracing::{span::{Id, Attributes, Record}, Metadata};

pub struct FooSubscriber;

impl tracing::Subscriber for FooSubscriber {
  fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
  fn record(&self, _: &Id, _: &Record) {}
  fn event(&self, _: &tracing::Event) {}
  fn record_follows_from(&self, _: &Id, _: &Id) {}
  fn enabled(&self, _: &Metadata) -> bool { false }
  fn enter(&self, _: &Id) {}
  fn exit(&self, _: &Id) {}
}

impl FooSubscriber {
  fn new() -> Self { FooSubscriber }
}

let my_subscriber = FooSubscriber::new();

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

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.

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

In addition, you can locally override the default subscriber, using the tokio pattern of executing code in a context. For example:

use tracing::{span::{Id, Attributes, Record}, Metadata};

pub struct FooSubscriber;

impl tracing::Subscriber for FooSubscriber {
  fn new_span(&self, _: &Attributes) -> Id { Id::from_u64(0) }
  fn record(&self, _: &Id, _: &Record) {}
  fn event(&self, _: &tracing::Event) {}
  fn record_follows_from(&self, _: &Id, _: &Id) {}
  fn enabled(&self, _: &Metadata) -> bool { false }
  fn enter(&self, _: &Id) {}
  fn exit(&self, _: &Id) {}
}

impl FooSubscriber {
  fn new() -> Self { FooSubscriber }
}

let my_subscriber = FooSubscriber::new();

tracing::subscriber::with_default(my_subscriber, || {
    // Any trace events generated in this closure or by functions it calls
    // will be collected by `my_subscriber`.
})

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 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.
  • 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.

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

~0.7–1MB
~22K SLoC