tracing 0.1.0

A scoped, structured logging and diagnostics system.


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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.


First, add this to your Cargo.toml:

tracing = "0.1"

Next, add this to your crate:

extern crate tracing;

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:

// Construct a new span named "my span".
let mut span = span!("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::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:

extern crate tracing;
use tracing::field;

pub fn shave_the_yak(yak: &mut Yak) {
    // Create a new span for this invocation of `shave_the_yak`, annotated
    // with  the yak being shaved as a *field* on the span.
    span!("shave_the_yak", yak = field::debug(&yak)).in_scope(|| {
        // 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 = field::display(razor) }, "Razor located");
                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);

You can find 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.

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

extern crate tracing;

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:

extern crate tracing;

let my_subscriber = FooSubscriber::new();

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

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.

The executable itself may use the tracing crate to instrument itself as well.

The tracing-nursery repository contains less stable crates designed to be used with the tracing ecosystem. It includes a collection of Subscriber implementations, as well as utility and adapter crates.


This project is licensed under the MIT license.


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.