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#![doc(html_root_url = "https://docs.rs/tracing/0.1.9")] #![deny(missing_debug_implementations, missing_docs, unreachable_pub)] #![cfg_attr(test, deny(warnings))] //! A scoped, structured logging and diagnostics system. //! //! # 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. //! //! # Core Concepts //! //! The core of `tracing`'s API is composed of _spans_, _events_ and //! _subscribers_. We'll cover these in turn. //! //! ## Spans //! //! To record the flow of execution through a program, `tracing` introduces the //! concept of [spans][span]. Unlike a log line that represents a _moment in //! time_, a span represents a _period of time_ with a beginning and an end. When a //! program begins executing in a context or performing a unit of work, it //! _enters_ that context's span, and when it stops executing in that context, //! it _exits_ the span. The span in which a thread is currently executing is //! referred to as that thread's _current_ span. //! //! For example: //! ``` //! use tracing::{span, Level}; //! # fn main() { //! 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][span]'s documentation provides further details on how to //! use spans. //! //! ## Events //! //! An [`Event`] represents a _moment_ in time. It signifies something that //! happened while a trace was being recorded. `Event`s 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. //! //! For example: //! ``` //! use tracing::{event, span, Level}; //! //! # fn main() { //! // records an event outside of any span context: //! event!(Level::INFO, "something happened"); //! //! let span = span!(Level::INFO, "my_span"); //! let _guard = span.enter(); //! //! // records an event within "my_span". //! event!(Level::DEBUG, "something happened inside my_span"); //! # } //!``` //! //! 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. //! //! The [`Event` struct][`Event`] documentation provides further details on using //! events. //! //! ## Subscribers //! //! As `Span`s and `Event`s occur, they are recorded or aggregated by //! implementations of the [`Subscriber`] trait. `Subscriber`s 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: //! //! + [`event`][Subscriber::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. //! //! # Usage //! //! First, add this to your `Cargo.toml`: //! //! ```toml //! [dependencies] //! tracing = "0.1" //! ``` //! //! ## Recording Spans and Events //! //! Spans and events are recorded using macros. //! //! ### Spans //! //! The [`span!`] macro expands to a [`Span` struct][`Span`] which is used to //! record a span. The [`Span::enter`] method on that struct records that the //! span has been entered, and returns a [RAII] guard object, which will exit //! the span when dropped. //! //! For example: //! //! ```rust //! use tracing::{span, Level}; //! # fn main() { //! // 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. //! # } //! ``` //! //! The [`#[instrument]`][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::{Level, event, 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`. //! event!(Level::INFO, "inside my_function!"); //! // ... //! } //! # fn main() {} //! ``` //! //! **Note**: using `#[instrument]` on `async fn`s requires the //! [`tracing-futures`] crate as a dependency, as well. //! //! You can find more examples showing how to use this crate [here][examples]. //! //! [RAII]: https://github.com/rust-unofficial/patterns/blob/master/patterns/RAII.md //! [examples]: https://github.com/tokio-rs/tracing/tree/master/tracing/examples //! //! ### Events //! //! [`Event`]s are recorded using the [`event!`] macro: //! //! ```rust //! # fn main() { //! use tracing::{event, Level}; //! event!(Level::INFO, "something has happened!"); //! # } //! ``` //! //! ## Using the Macros //! //! The [`span!`] and [`event!`] macros use fairly similar syntax, with some //! exceptions. //! //! ### Configuring Attributes //! //! Both macros require a [`Level`] specifying the verbosity of the span or //! event. Optionally, the [target] and [parent span] may be overridden. If the //! target and parent span are not overridden, they will default to the //! module path where the macro was invoked and the current span (as determined //! by the subscriber), respectively. //! //! For example: //! //! ``` //! # use tracing::{span, event, Level}; //! # fn main() { //! span!(target: "app_spans", Level::TRACE, "my span"); //! event!(target: "app_events", Level::INFO, "something has happened!"); //! # } //! ``` //! ``` //! # use tracing::{span, event, Level}; //! # fn main() { //! let span = span!(Level::TRACE, "my span"); //! event!(parent: &span, Level::INFO, "something has happened!"); //! # } //! ``` //! //! The span macros also take a string literal after the level, to set the name //! of the span. //! //! ### Recording Fields //! //! Structured fields on spans and events are specified using the syntax //! `field_name = field_value`. Fields are separated by commas. //! //! ``` //! # use tracing::{event, Level}; //! # fn main() { //! // records an event with two fields: //! // - "answer", with the value 42 //! // - "question", with the value "life, the universe and everything" //! event!(Level::INFO, answer = 42, question = "life, the universe, and everything"); //! # } //! ``` //! //! As shorthand, local variables may be used as field values without an //! assignment, similar to [struct initializers]. For example: //! //! ``` //! # use tracing::{span, Level}; //! # fn main() { //! let user = "ferris"; //! //! span!(Level::TRACE, "login", user); //! // is equivalent to: //! span!(Level::TRACE, "login", user = user); //! # } //!``` //! //! Field names can include dots, but should not be terminated by them: //! ``` //! # use tracing::{span, Level}; //! # fn main() { //! let user = "ferris"; //! let email = "ferris@rust-lang.org"; //! span!(Level::TRACE, "login", user, user.email = email); //! # } //!``` //! //! Since field names can include dots, fields on local structs can be used //! using the local variable shorthand: //! ``` //! # use tracing::{span, Level}; //! # fn main() { //! # struct User { //! # name: &'static str, //! # email: &'static str, //! # } //! let user = User { //! name: "ferris", //! email: "ferris@rust-lang.org", //! }; //! // the span will have the fields `user.name = "ferris"` and //! // `user.email = "ferris@rust-lang.org"`. //! span!(Level::TRACE, "login", user.name, user.email); //! # } //!``` //! //! The `?` sigil is shorthand that specifies a field should be recorded using //! its [`fmt::Debug`] implementation: //! ``` //! # use tracing::{event, Level}; //! # fn main() { //! #[derive(Debug)] //! struct MyStruct { //! field: &'static str, //! } //! //! let my_struct = MyStruct { //! field: "Hello world!" //! }; //! //! // `my_struct` will be recorded using its `fmt::Debug` implementation. //! event!(Level::TRACE, greeting = ?my_struct); //! // is equivalent to: //! event!(Level::TRACE, greeting = tracing::field::debug(&my_struct)); //! # } //! ``` //! //! The `%` sigil operates similarly, but indicates that the value should be //! recorded using its [`fmt::Display`] implementation: //! ``` //! # use tracing::{event, Level}; //! # fn main() { //! # #[derive(Debug)] //! # struct MyStruct { //! # field: &'static str, //! # } //! # //! # let my_struct = MyStruct { //! # field: "Hello world!" //! # }; //! // `my_struct.field` will be recorded using its `fmt::Display` implementation. //! event!(Level::TRACE, greeting = %my_struct.field); //! // is equivalent to: //! event!(Level::TRACE, greeting = tracing::field::display(&my_struct.field)); //! # } //! ``` //! //! The `%` and `?` sigils may also be used with local variable shorthand: //! //! ``` //! # use tracing::{event, Level}; //! # fn main() { //! # #[derive(Debug)] //! # struct MyStruct { //! # field: &'static str, //! # } //! # //! # let my_struct = MyStruct { //! # field: "Hello world!" //! # }; //! // `my_struct.field` will be recorded using its `fmt::Display` implementation. //! event!(Level::TRACE, %my_struct.field); //! # } //! ``` //! //! Note that a span may have up to 32 fields. The following will not compile: //! //! ```rust,compile_fail //! # use tracing::Level; //! # fn main() { //! let bad_span = span!( //! Level::TRACE, //! "too many fields!", //! a = 1, b = 2, c = 3, d = 4, e = 5, f = 6, g = 7, h = 8, i = 9, //! j = 10, k = 11, l = 12, m = 13, n = 14, o = 15, p = 16, q = 17, //! r = 18, s = 19, t = 20, u = 21, v = 22, w = 23, x = 24, y = 25, //! z = 26, aa = 27, bb = 28, cc = 29, dd = 30, ee = 31, ff = 32, gg = 33 //! ); //! # } //! ``` //! //! Finally, events may also include human-readable messages, in the form of a //! [format string][fmt] and (optional) arguments, **after** the event's //! key-value fields. If a format string and arguments are provided, //! they will implicitly create a new field named `message` whose value is the //! provided set of format arguments. //! //! For example: //! //! ``` //! # use tracing::{event, Level}; //! # fn main() { //! let question = "the answer to the ultimate question of life, the universe, and everything"; //! let answer = 42; //! // records an event with the following fields: //! // - `question.answer` with the value 42, //! // - `question.tricky` with the value `true`, //! // - "message", with the value "the answer to the ultimate question of life, the //! // universe, and everthing is 42." //! event!( //! Level::DEBUG, //! question.answer = answer, //! question.tricky = true, //! "the answer to {} is {}.", question, answer //! ); //! # } //! ``` //! //! Specifying a formatted message in this manner does not allocate by default. //! //! [struct initializers]: https://doc.rust-lang.org/book/ch05-01-defining-structs.html#using-the-field-init-shorthand-when-variables-and-fields-have-the-same-name //! [target]: struct.Metadata.html#method.target //! [parent span]: span/struct.Attributes.html#method.parent //! [determined contextually]: span/struct.Attributes.html#method.is_contextual //! [`fmt::Debug`]: https://doc.rust-lang.org/std/fmt/trait.Debug.html //! [`fmt::Display`]: https://doc.rust-lang.org/std/fmt/trait.Display.html //! [fmt]: https://doc.rust-lang.org/std/fmt/#usage //! //! ### Shorthand Macros //! //! `tracing` also offers a number of macros with preset verbosity levels. //! The [`trace!`], [`debug!`], [`info!`], [`warn!]`, and [`error!`] behave //! similarly to the [`event!`] macro, but with the [`Level`] argument already //! specified, while the corresponding [`trace_span!`], [`debug_span!`], //! [`info_span!`], [`warn_span!`], and [`error_span!`] macros are the same, //! but for the `[span!`] macro. //! //! These are intended both as a shorthand, and for compatibility with the [`log`] //! crate (see the next section). //! //! [`span!`]: macro.span.html //! [`event!`]: macro.event.html //! [`trace!`]: macro.trace.html //! [`debug!`]: macro.debug.html //! [`info!`]: macro.info.html //! [`warn!`]: macro.warn.html //! [`error!`]: macro.error.html //! [`trace_span!`]: macro.trace_span.html //! [`debug_span!`]: macro.debug_span.html //! [`info_span!`]: macro.info_span.html //! [`warn_span!`]: macro.warn_span.html //! [`error_span!`]: macro.error_span.html //! [`Level`]: struct.Level.html //! //! ### For `log` Users //! //! Users of the [`log`] crate should note that `tracing` exposes a set of //! macros for creating `Event`s (`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: //! //! ```rust //! use tracing::{info, span, warn, Level}; //! //! # #[derive(Debug)] pub struct Yak(String); //! # impl Yak { fn shave(&mut self, _: u32) {} } //! # fn find_a_razor() -> Result<u32, u32> { Ok(1) } //! # fn main() { //! 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"); //! 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); //! } //! } //! } //! } //! # } //! ``` //! //! ## 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. //! //! This library does not contain any `Subscriber` implementations; these are //! provided by [other crates](#related-crates). //! //! The simplest way to use a subscriber is to call the [`set_global_default`] //! function: //! //! ``` //! extern crate tracing; //! # pub struct FooSubscriber; //! # use tracing::{span::{Id, Attributes, Record}, Metadata}; //! # 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 } //! # } //! # fn main() { //! //! let my_subscriber = FooSubscriber::new(); //! tracing::subscriber::set_global_default(my_subscriber) //! .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: //! //! ```rust //! # pub struct FooSubscriber; //! # use tracing::{span::{Id, Attributes, Record}, Metadata}; //! # 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 } //! # } //! # fn main() { //! //! let my_subscriber = FooSubscriber::new(); //! # #[cfg(feature = "std")] //! 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. //! //! ## 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 `Subscriber`s 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 `Future`s, `Stream`s, and `Executor`s. //! - [`tracing-subscriber`] provides `Subscriber` implementations and //! utilities for working with `Subscriber`s. 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` `Event`s 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, if a default `tracing` subscriber has not been set. 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. //! * `log-always`: Emit `log` records from all `tracing` spans and events, even //! a `tracing` subscriber has been set. This should be set only by //! applications which intend to collect traces and logs separately; if an //! adapter is used to convert `log` records into `tracing` events, this will //! cause duplicate events to occur. //! * `std`: Depend on the Rust standard library (enabled by default). //! //! `no_std` users may disable this feature with `default-features = false`: //! //! ```toml //! [dependencies] //! tracing = { version = "0.1.9", default-features = false } //! ``` //! **Note**:`tracing`'s `no_std` support requires `liballoc`. //! //! [`log`]: https://docs.rs/log/0.4.6/log/ //! [span]: span/index.html //! [`Span`]: span/struct.Span.html //! [`in_scope`]: span/struct.Span.html#method.in_scope //! [`Event`]: struct.Event.html //! [`Subscriber`]: subscriber/trait.Subscriber.html //! [Subscriber::event]: subscriber/trait.Subscriber.html#tymethod.event //! [`enter`]: subscriber/trait.Subscriber.html#tymethod.enter //! [`exit`]: subscriber/trait.Subscriber.html#tymethod.exit //! [`enabled`]: subscriber/trait.Subscriber.html#tymethod.enabled //! [metadata]: struct.Metadata.html //! [`field::display`]: field/fn.display.html //! [`field::debug`]: field/fn.debug.html //! [`set_global_default`]: subscriber/fn.set_global_default.html //! [`with_default`]: subscriber/fn.with_default.html //! [`tokio-rs/tracing`]: https://github.com/tokio-rs/tracing //! [`tracing-futures`]: https://crates.io/crates/tracing-futures //! [`tracing-subscriber`]: https://crates.io/crates/tracing-subscriber //! [`tracing-log`]: https://crates.io/crates/tracing-log //! [`tracing-timing`]: https://crates.io/crates/tracing-timing //! [`env_logger`]: https://crates.io/crates/env_logger //! [`FmtSubscriber`]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/fmt/struct.Subscriber.html //! [static verbosity level]: level_filters/index.html#compile-time-filters //! [instrument]: https://docs.rs/tracing-attributes/latest/tracing_attributes/attr.instrument.html #![cfg_attr(not(feature = "std"), no_std)] #[cfg(not(feature = "std"))] extern crate alloc; #[macro_use] extern crate cfg_if; use tracing_core; #[cfg(feature = "log")] #[doc(hidden)] pub extern crate log; // Somehow this `use` statement is necessary for us to re-export the `core` // macros on Rust 1.26.0. I'm not sure how this makes it work, but it does. #[allow(unused_imports)] #[doc(hidden)] use tracing_core::*; pub use self::{ dispatcher::Dispatch, event::Event, field::Value, subscriber::Subscriber, tracing_core::{dispatcher, event, Level, Metadata}, }; #[doc(hidden)] pub use self::{ span::Id, tracing_core::{ callsite::{self, Callsite}, metadata, }, }; #[doc(inline)] pub use self::span::Span; #[doc(inline)] pub use tracing_attributes::instrument; #[macro_use] mod macros; pub mod field; pub mod level_filters; pub mod span; pub(crate) mod stdlib; pub mod subscriber; #[doc(hidden)] pub mod __macro_support { pub use crate::stdlib::sync::atomic::{AtomicUsize, Ordering}; #[cfg(feature = "std")] pub use crate::stdlib::sync::Once; #[cfg(not(feature = "std"))] pub type Once = spin::Once<()>; } mod sealed { pub trait Sealed {} }