[][src]Crate tracing

A scoped, structured logging and diagnostics system.


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.

Core Concepts

The core of tracing's API is composed of spans, events and subscribers. We'll cover these in turn.


A span represents a period of time during which a program was executing in some context. A thread of execution is said to enter a span when it begins executing in that context, and to exit the span when switching to another context. The span in which a thread is currently executing is referred to as the current span.

For example:

extern crate tracing;

use tracing::Level;

let span = span!(Level::TRACE, "my_span");
// `enter` returns a RAII guard which, when dropped, exits the span. this
// indicates that we are in the span for the current lexical scope.
let _enter = span.enter();
// perform some work in the context of `my_span`...

The span module's documentation provides further details on how to use spans.


An Event represents a point in time. It signifies something that happened while the trace was executing. Events are comparable to the log records emitted by unstructured logging code, but unlike a typical log line, an Event may occur within the context of a Span. Like a Span, it may have fields, and implicitly inherits any of the fields present on its parent span.

For example:

// records an event outside of any span context:
event!(Level::INFO, "something happened");

span!(Level::INFO, "my_span").in_scope(|| {
    // records an event within "my_span".
    event!(Level::DEBUG, "something happened inside my_span");

Essentially, Events bridge the gap between traditional unstructured logging and span-based tracing. Similar to log records, they may be recorded at a number of levels, and can have unstructured, human-readable messages; however, they also carry key-value data and exist within the context of the tree of spans that comprise a trace. Thus, individual log record-like events can be pinpointed not only in time, but in the logical execution flow of the system.

In general, 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.


As Spans and Events occur, they are recorded or aggregated by implementations of the Subscriber trait. Subscribers are notified when an Event takes place and when a Span is entered or exited. These notifications are represented by the following Subscriber trait methods: + observe_event, called when an Event takes place, + enter, called when execution enters a Span, + exit, called when execution exits a Span

In addition, subscribers may implement the enabled function to filter the notifications they receive based on metadata describing each Span or Event. If a call to Subscriber::enabled returns false for a given set of metadata, that Subscriber will not be notified about the corresponding Span or Event. For performance reasons, if no currently active subscribers express interest in a given set of metadata by returning true, then the corresponding Span or Event will never be constructed.


First, add this to your Cargo.toml:

tracing = "0.1"

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" with trace log level.
let span = span!(Level::TRACE, "my span");

// Enter the span, returning a guard object.
let _enter = span.enter();

// Any trace events that occur before the guard is dropped will occur
// within the span.

// Dropping the guard will exit the span.

Events are created using the event! macro, and are recorded when the event is dropped:

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:

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

pub fn shave_the_yak(yak: &mut Yak) {
    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");
            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);

The #[instrument] attribute provides an easy way to add tracing spans to functions. A function annotated with #[instrument] will create and enter a span with that function's name every time the function is called, with arguments to that function will be recorded as fields using fmt::Debug.

For example:

use tracing::{info, instrument};

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

You can find more 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.

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

extern crate tracing;

let my_subscriber = FooSubscriber::new();
    .expect("setting tracing default failed");

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

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

In addition, the default subscriber can be set through using the with_default function. This follows the tokio pattern of using closures to represent executing code in a context that is exited at the end of the closure. For example:

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`.

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 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-fmt provides a Subscriber implementation 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.

Crate Feature Flags

The following crate feature flags are available:

  • A set of features controlling the static verbosity level.
  • log: causes trace instrumentation points to emit log records as well as trace events. This is intended for use in libraries whose users may be using either tracing or log. Note: log support will not work when tracing is renamed in Cargo.toml, due to oddities in macro expansion.
  • async-await: enables support for instrumenting async fns with the #[instrument] attribute. Note: this also requires the tracing-futures crate with the std-future feature flag enabled.
tracing = { version = "0.1", features = ["log", "async-await"] }


pub use tracing_attributes::instrument;



Dispatches trace events to Subscribers.


Events represent single points in time during the execution of a program.


Structured data associated with Spans and Events.


Trace verbosity level filtering.


Spans represent periods of time in which a program was executing in a particular context.


Collects and records trace data.



Constructs an event at the debug level.


Constructs a span at the debug level.


Constructs an event at the error level.


Constructs a span at the error level.


Constructs a new Event.


Constructs an event at the info level.


Constructs a span at the info level.


Constructs a new span.


Constructs an event at the trace level.


Constructs a span at the trace level.


Constructs an event at the warn level.


Constructs a span at the warn level.



Dispatch trace data to a Subscriber.


Events represent single points in time where something occurred during the execution of a program.


Describes the level of verbosity of a span or event.


Metadata describing a span or event.


A handle representing a span, with the capability to enter the span if it exists.



Trait representing the functions required to collect trace data.


A field value of an erased type.