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#![doc(html_root_url = "https://docs.rs/tracing/0.1.6")] #![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 //! //! 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: //! ``` //! 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]'s documentation provides further details on how to use spans. //! //! ## Events //! //! An [`Event`] represents a _point_ in time. It signifies something that //! happened while the trace was executing. `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`. Like a `Span`, it //! may have fields, and implicitly inherits any of the fields present on its //! parent 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"); //! # } //!``` //! //! Essentially, `Event`s 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. //! //! ## `Subscriber`s //! //! 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: //! + [`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. //! //! # Usage //! //! First, add this to your `Cargo.toml`: //! //! ```toml //! [dependencies] //! tracing = "0.1" //! ``` //! `Span`s are constructed using the `span!` macro, and then _entered_ //! to indicate that some code takes place within the context of that `Span`: //! //! ```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. //! # } //! ``` //! //! `Event`s are created using the `event!` macro, and are recorded when the //! event is dropped: //! //! ```rust //! # fn main() { //! 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 `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); //! } //! } //! } //! } //! # } //! ``` //! //! 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::{info, 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`. //! 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 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; //! # 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. //! //! 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-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` `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. 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. //! * `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 = "0.1.5", 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` module]: span/index.html //! [`in_scope`]: span/struct.Span.html#method.in_scope //! [`Event`]: struct.Event.html //! [`Subscriber`]: subscriber/trait.Subscriber.html //! [`observe_event`]: subscriber/trait.Subscriber.html#tymethod.observe_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://github.com/tokio-rs/tracing/tree/master/tracing-futures //! [`tracing-fmt`]: https://github.com/tokio-rs/tracing/tree/master/tracing-fmt //! [`tracing-log`]: https://github.com/tokio-rs/tracing/tree/master/tracing-log //! [`tracing-timing`]: https://crates.io/crates/tracing-timing //! [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 {} }