tracing/README.md

tokio-trace

A scoped, structured logging and diagnostics system.

Overview

tokio-trace 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. tokio-trace 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 tokio-trace 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, tokio-trace spans are structured, with the ability to record typed data as well as textual messages.

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

Usage

First, add this to your Cargo.toml:

[dependencies]
tokio-trace = { git = "https://github.com/tokio-rs/tokio" }

Next, add this to your crate:

#[macro_use]
extern crate tokio_trace;

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.enter(|| {
    // 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 tokio_trace::Level;
event!(Level::INFO, "something has happened!");

Users of the log crate should note that tokio-trace 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 tokio-trace can begin with a simple drop-in replacement.

Let's consider the log crate's yak-shaving example:

#[macro_use]
extern crate tokio_trace;
use tokio_trace::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)).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 = field::display(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);
                }
            }
        }
    })
}

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

In libraries

Libraries should link only to the tokio-trace 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 tokio-trace. A Subscriber implements a way of collecting trace data, such as by logging it to standard output.

Unlike the log crate, tokio-trace does not use a global Subscriber which is initialized once. Instead, it follows the tokio pattern of executing code in a context. For example:

#[macro_use]
extern crate tokio_trace;

let my_subscriber = FooSubscriber::new();

tokio_trace::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. Alternatively, a single subscriber may be constructed by the main function and all subsequent code executed with that subscriber as the default. Any trace events generated outside the context of a subscriber will not be collected.

The executable itself may use the tokio-trace crate to instrument itself as well.

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

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.