Module tracing_subscriber::subscribe

The Subscribe trait, a composable abstraction for building collectors.

The Collect trait in tracing-core represents the complete set of functionality required to consume tracing instrumentation. This means that a single Collect instance is a self-contained implementation of a complete strategy for collecting traces; but it also means that the Collect trait cannot easily be composed with other collectors.

In particular, collectors are responsible for generating span IDs and assigning them to spans. Since these IDs must uniquely identify a span within the context of the current trace, this means that there may only be a single collector for a given thread at any point in time — otherwise, there would be no authoritative source of span IDs.

On the other hand, the majority of the Collect trait’s functionality is composable: any number of subscribers may observe events, span entry and exit, and so on, provided that there is a single authoritative source of span IDs. The Subscribe trait represents this composable subset of the Collect behavior; it can observe events and spans, but does not assign IDs.

Composing Subscribers

Since a subscriber does not implement a complete strategy for collecting traces, it must be composed with a collector in order to be used. The Subscribe trait is generic over a type parameter (called C in the trait definition), representing the types of Collect they can be composed with. Thus, a subscriber may be implemented that will only compose with a particular Collect implementation, or additional trait bounds may be added to constrain what types implementing Collect a subscriber can wrap.

Subscribers may be added to a collector by using the CollectExt::with method, which is provided by tracing-subscriber’s prelude. This method returns a Layered struct that implements Collect by composing the Subscribe with the collector.

For example:

use tracing_subscriber::Subscribe;
use tracing_subscriber::prelude::*;
use tracing::Collect;

pub struct MySubscriber {
    // ...
}

impl<C: Collect> Subscribe<C> for MySubscriber {
    // ...
}

pub struct MyCollector {
    // ...
}

impl Collect for MyCollector {
    // ...
}

let collector = MyCollector::new()
    .with(MySubscriber::new());

tracing::collect::set_global_default(collector);

Multiple subscriber may be composed in the same manner:

pub struct MyOtherSubscriber {
    // ...
}

impl<C: Collect> Subscribe<C> for MyOtherSubscriber {
    // ...
}

pub struct MyThirdSubscriber {
    // ...
}

impl<C: Collect> Subscribe<C> for MyThirdSubscriber {
    // ...
}
}

let collect = MyCollector::new()
    .with(MySubscriber::new())
    .with(MyOtherSubscriber::new())
    .with(MyThirdSubscriber::new());

tracing::collect::set_global_default(collect);

The Subscribe::with_collector constructs the Layered type from a Subscribe and Collect, and is called by CollectExt::with. In general, it is more idiomatic to use CollectExt::with, and treat Subscribe::with_collector as an implementation detail, as with_collector calls must be nested, leading to less clear code for the reader.

Runtime Configuration With Subscribers

In some cases, a particular subscriber may be enabled or disabled based on runtime configuration. This can introduce challenges, because the type of a layered collector depends on which subscribers are added to it: if an if or match expression adds some Subscribe implementation in one branch, and other subscribers in another, the collector values returned by those branches will have different types. For example, the following will not work:

use std::fs::File;
use tracing_subscriber::{Registry, prelude::*};

let stdout_log = tracing_subscriber::fmt::subscriber().pretty();
let collector = Registry::default().with(stdout_log);

// The compile error will occur here because the if and else
// branches have different (and therefore incompatible) types.
let collector = if cfg.is_prod {
    let file = File::create(cfg.path)?;
    let collector = tracing_subscriber::fmt::subscriber()
        .json()
        .with_writer(Arc::new(file));
    collector.with(subscriber)
} else {
    collector
};

tracing::collect::set_global_default(collector)
    .expect("Unable to set global collector");

However, a Subscribe wrapped in an Option also implements the Subscribe trait. This allows individual layers to be enabled or disabled at runtime while always producing a Collect of the same type. For example:

use std::fs::File;
use tracing_subscriber::{Registry, prelude::*};

let stdout_log = tracing_subscriber::fmt::subscriber().pretty();
let collector = Registry::default().with(stdout_log);

// if `cfg.is_prod` is true, also log JSON-formatted logs to a file.
let json_log = if cfg.is_prod {
    let file = File::create(cfg.path)?;
    let json_log = tracing_subscriber::fmt::subscriber()
        .json()
        .with_writer(file);
    Some(json_log)
} else {
    None
};

// If `cfg.is_prod` is false, then `json` will be `None`, and this subscriber
// will do nothing. However, the collector will still have the same type
// regardless of whether the `Option`'s value is `None` or `Some`.
let collector = collector.with(json_log);

tracing::collect::set_global_default(collector)
   .expect("Unable to set global collector");

If a subscriber may be one of several different types, note that Box<dyn Subscribe<C> + Send + Sync + 'static> implements Subscribe. This may be used to erase the type of a subscriber.

For example, a function that configures a subscriber to log to one of several outputs might return a Box<dyn Subscribe<C> + Send + Sync + 'static>:

use tracing_subscriber::{
    Subscribe,
    registry::LookupSpan,
    prelude::*,
};
use std::{path::PathBuf, fs::File, io};

/// Configures whether logs are emitted to a file, to stdout, or to stderr.
pub enum LogConfig {
    File(PathBuf),
    Stdout,
    Stderr,
}

impl LogConfig {
    pub fn subscriber<C>(self) -> Box<dyn Subscribe<C> + Send + Sync + 'static>
    where
        C: tracing_core::Collect,
        for<'a> C: LookupSpan<'a>,
    {
        // Shared configuration regardless of where logs are output to.
        let fmt = tracing_subscriber::fmt::subscriber()
            .with_target(true)
            .with_thread_names(true);

        // Configure the writer based on the desired log target:
        match self {
            LogConfig::File(path) => {
                let file = File::create(path).expect("failed to create log file");
                Box::new(fmt.with_writer(file))
            },
            LogConfig::Stdout => Box::new(fmt.with_writer(io::stdout)),
            LogConfig::Stderr => Box::new(fmt.with_writer(io::stderr)),
        }
    }
}

let config = LogConfig::Stdout;
tracing_subscriber::registry()
    .with(config.subscriber())
    .init();

The Subscribe::boxed method is provided to make boxing a subscriber more convenient, but Box::new may be used as well.

When the number of subscribers varies at runtime, note that a Vec<S> where S: Subscribe also implements Subscribe. This can be used to add a variable number of subscribers to a collector:

use tracing_subscriber::{Subscribe, prelude::*};
struct MySubscriber {
    // ...
}

impl<C: tracing_core::Collect> Subscribe<C> for MySubscriber {
    // ...
}

/// Returns how many subscribers we need
fn how_many_subscribers() -> usize {
    // ...
}

// Create a variable-length `Vec` of subscribers
let mut subscribers = Vec::new();
for _ in 0..how_many_subscribers() {
    subscribers.push(MySubscriber::new());
}

tracing_subscriber::registry()
    .with(subscribers)
    .init();

If a variable number of subscribers is needed and those subscribers have different types, a Vec of boxed subscriber trait objects may be used. For example:

use tracing_subscriber::{filter::LevelFilter, Subscribe, prelude::*};
use std::fs::File;
struct Config {
    enable_log_file: bool,
    enable_stdout: bool,
    enable_stderr: bool,
    // ...
}

let cfg = Config::from_config_file()?;

// Based on our dynamically loaded config file, create any number of subscribers:
let mut subscribers = Vec::new();

if cfg.enable_log_file {
    let file = File::create("myapp.log")?;
    let subscriber = tracing_subscriber::fmt::subscriber()
        .with_thread_names(true)
        .with_target(true)
        .json()
        .with_writer(file)
        // Box the subscriber as a type-erased trait object, so that it can
        // be pushed to the `Vec`.
        .boxed();
    subscribers.push(subscriber);
}

if cfg.enable_stdout {
    let subscriber = tracing_subscriber::fmt::subscriber()
        .pretty()
        .with_filter(LevelFilter::INFO)
        // Box the subscriber as a type-erased trait object, so that it can
        // be pushed to the `Vec`.
        .boxed();
    subscribers.push(subscriber);
}

if cfg.enable_stdout {
    let subscriber = tracing_subscriber::fmt::subscriber()
        .with_target(false)
        .with_filter(LevelFilter::WARN)
        // Box the subscriber as a type-erased trait object, so that it can
        // be pushed to the `Vec`.
        .boxed();
    subscribers.push(subscriber);
}

tracing_subscriber::registry()
    .with(subscribers)
    .init();

Finally, if the number of subscribers changes at runtime, a Vec of subscribers can be used alongside the reload module to add or remove subscribers dynamically at runtime.

Recording Traces

The Subscribe trait defines a set of methods for consuming notifications from tracing instrumentation, which are generally equivalent to the similarly named methods on Collect. Unlike Collect, the methods on Subscribe are additionally passed a Context type, which exposes additional information provided by the wrapped subscriber (such as the current span) to the subscriber.

Filtering with Subscribers

As well as strategies for handling trace events, the Subscribe trait may also be used to represent composable filters. This allows the determination of what spans and events should be recorded to be decoupled from how they are recorded: a filtering subscriber can be applied to other subscribers or subscribers. Subscribes can be used to implement global filtering, where a Subscribe provides a filtering strategy for the entire subscriber. Additionally, individual recording Subscribes or sets of Subscribes may be combined with per-subscriber filters that control what spans and events are recorded by those subscribers.

Global Filtering

A Subscribe that implements a filtering strategy should override the register_callsite and/or enabled methods. It may also choose to implement methods such as on_enter, if it wishes to filter trace events based on the current span context.

Note that the Subscribe::register_callsite and Subscribe::enabled methods determine whether a span or event is enabled globally. Thus, they should not be used to indicate whether an individual subscriber wishes to record a particular span or event. Instead, if a subscriber is only interested in a subset of trace data, but does not wish to disable other spans and events for the rest of the subscriber stack should ignore those spans and events in its notification methods.

The filtering methods on a stack of Subscribes are evaluated in a top-down order, starting with the outermost Subscribe and ending with the wrapped Collect. If any subscriber returns false from its enabled method, or Interest::never() from its register_callsite method, filter evaluation will short-circuit and the span or event will be disabled.

Enabling Interest

Whenever an tracing event (or span) is emitted, it goes through a number of steps to determine how and how much it should be processed. The earlier an event is disabled, the less work has to be done to process the event, so subscribers that implement filtering should attempt to disable unwanted events as early as possible. In order, each event checks:

• register_callsite, once per callsite (roughly: once per time that event! or span! is written in the source code; this is cached at the callsite). See Collect::register_callsite and tracing_core::callsite for a summary of how this behaves.
• enabled, once per emitted event (roughly: once per time that event! or span! is executed), and only if register_callsite registers an Interest::sometimes. This is the main customization point to globally filter events based on their Metadata. If an event can be disabled based only on Metadata, it should be, as this allows the construction of the actual Event/Span to be skipped.
• For events only (and not spans), event_enabled is called just before processing the event. This gives subscribers one last chance to say that an event should be filtered out, now that the event’s fields are known.

Per-Subscriber Filtering

Note: per-subscriber filtering APIs currently require the "registry" crate feature flag to be enabled.

Sometimes, it may be desirable for one Subscribe to record a particular subset of spans and events, while a different subset of spans and events are recorded by other Subscribes. For example:

• A subscriber that records metrics may wish to observe only events including particular tracked values, while a logging subscriber ignores those events.
• If recording a distributed trace is expensive, it might be desirable to only send spans with INFO and lower verbosity to the distributed tracing system, while logging more verbose spans to a file.
• Spans and events with a particular target might be recorded differently from others, such as by generating an HTTP access log from a span that tracks the lifetime of an HTTP request.

The Filter trait is used to control what spans and events are observed by an individual Subscribe, while still allowing other Subscribes to potentially record them. The Subscribe::with_filter method combines a Subscribe with a Filter, returning a Filtered subscriber.

This crate’s filter module provides a number of types which implement the Filter trait, such as LevelFilter, Targets, and FilterFn. These Filters provide ready-made implementations of common forms of filtering. For custom filtering policies, the FilterFn and DynFilterFn types allow implementing a Filter with a closure or function pointer. In addition, when more control is required, the Filter trait may also be implemented for user-defined types.

Option<Filter> also implements Filter, which allows for an optional filter. None filters out nothing (that is, allows everything through). For example:

fn setup_tracing<C: Collect>(filter_config: Option<&str>) {
    let layer = MySubscriber::<C>::new()
        .with_filter(filter_config.map(|config| filter_fn(my_filter(config))));
//...
}

     Warning: Currently, the 
     Registry type defined in this crate is the only root
     Collect capable of supporting subscriberss with
     per-subscriber filters. In the future, new APIs will be added to allow other
     root Collects to support per-subscriber filters.
 

For example, to generate an HTTP access log based on spans with the http_access target, while logging other spans and events to standard out, a Filter can be added to the access log subscriber:

use tracing_subscriber::{filter, prelude::*};

// Generates an HTTP access log.
let access_log = // ...

// Add a filter to the access log subscriber so that it only observes
// spans and events with the `http_access` target.
let access_log = access_log.with_filter(filter::filter_fn(|metadata| {
    // Returns `true` if and only if the span or event's target is
    // "http_access".
    metadata.target() == "http_access"
}));

// A general-purpose logging subscriber.
let fmt_subscriber = tracing_subscriber::fmt::subscriber();

// Build a subscriber that combines the access log and stdout log
// subscribers.
tracing_subscriber::registry()
    .with(fmt_subscriber)
    .with(access_log)
    .init();

Multiple subscribers can have their own, separate per-subscriber filters. A span or event will be recorded if it is enabled by any per-subscriber filter, but it will be skipped by the subscribers whose filters did not enable it. Building on the previous example:

use tracing_subscriber::{filter::{filter_fn, LevelFilter}, prelude::*};

let access_log = // ...
let fmt_subscriber = tracing_subscriber::fmt::subscriber();

tracing_subscriber::registry()
    // Add the filter for the "http_access" target to the access
    // log subscriber, like before.
    .with(access_log.with_filter(filter_fn(|metadata| {
        metadata.target() == "http_access"
    })))
    // Add a filter for spans and events with the INFO level
    // and below to the logging subscriber.
    .with(fmt_subscriber.with_filter(LevelFilter::INFO))
    .init();

// Neither subscriber will observe this event
tracing::debug!(does_anyone_care = false, "a tree fell in the forest");

// This event will be observed by the logging subscriber, but not
// by the access log subscriber.
tracing::warn!(dose_roentgen = %3.8, "not great, but not terrible");

// This event will be observed only by the access log subscriber.
tracing::trace!(target: "http_access", "HTTP request started");

// Both subscribers will observe this event.
tracing::error!(target: "http_access", "HTTP request failed with a very bad error!");

A per-subscriber filter can be applied to multiple Subscribes at a time, by combining them into a Layered subscriber using Subscribe::and_then, and then calling Subscribe::with_filter on the resulting Layered subscriber.

Consider the following:

• subscriber_a and subscriber_b, which should only receive spans and events at the INFO level and above.
• A third subscriber, subscriber_c, which should receive spans and events at the DEBUG level as well. The subscribers and filters would be composed thusly:

use tracing_subscriber::{filter::LevelFilter, prelude::*};

let subscriber_a = // ...
let subscriber_b =  // ...
let subscriber_c =  // ...

let info_subscribers = subscriber_a
    // Combine `subscriber_a` and `subscriber_b` into a `Layered` subscriber:
    .and_then(subscriber_b)
    // ...and then add an `INFO` `LevelFilter` to that subscriber:
    .with_filter(LevelFilter::INFO);

tracing_subscriber::registry()
    // Add `subscriber_c` with a `DEBUG` filter.
    .with(subscriber_c.with_filter(LevelFilter::DEBUG))
    .with(info_subscribers)
    .init();

If a Filtered Subscribe is combined with another Subscribe Subscribe::and_then, and a filter is added to the Layered subscriber, that subscriber will be filtered by both the inner filter and the outer filter. Only spans and events that are enabled by both filters will be observed by that subscriber. This can be used to implement complex filtering trees.

As an example, consider the following constraints:

• Suppose that a particular target is used to indicate events that should be counted as part of a metrics system, which should be only observed by a subscriber that collects metrics.
• A log of high-priority events (INFO and above) should be logged to stdout, while more verbose events should be logged to a debugging log file.
• Metrics-focused events should not be included in either log output.

In that case, it is possible to apply a filter to both logging subscribers to exclude the metrics events, while additionally adding a LevelFilter to the stdout log:

use tracing_subscriber::{filter, prelude::*};
use std::{fs::File, sync::Arc};

// A subscriber that logs events to stdout using the human-readable "pretty"
// format.
let stdout_log = tracing_subscriber::fmt::subscriber()
    .pretty();

// A subscriber that logs events to a file.
let file = File::create("debug.log")?;
let debug_log = tracing_subscriber::fmt::subscriber()
    .with_writer(file);

// A subscriber that collects metrics using specific events.
let metrics_subscriber = /* ... */ filter::LevelFilter::INFO;

tracing_subscriber::registry()
    .with(
        stdout_log
            // Add an `INFO` filter to the stdout logging subscriber
            .with_filter(filter::LevelFilter::INFO)
            // Combine the filtered `stdout_log` subscriber with the
            // `debug_log` subscriber, producing a new `Layered` subscriber.
            .and_then(debug_log)
            // Add a filter to *both* subscribers that rejects spans and
            // events whose targets start with `metrics`.
            .with_filter(filter::filter_fn(|metadata| {
                !metadata.target().starts_with("metrics")
            }))
    )
    .with(
        // Add a filter to the metrics label that *only* enables
        // events whose targets start with `metrics`.
        metrics_subscriber.with_filter(filter::filter_fn(|metadata| {
            metadata.target().starts_with("metrics")
        }))
    )
    .init();

// This event will *only* be recorded by the metrics subscriber.
tracing::info!(target: "metrics::cool_stuff_count", value = 42);

// This event will only be seen by the debug log file subscriber:
tracing::debug!("this is a message, and part of a system of messages");

// This event will be seen by both the stdout log subscriber *and*
// the debug log file subscriber, but not by the metrics subscriber.
tracing::warn!("the message is a warning about danger!");

Structs

• Context
  Represents information about the current context provided to subscribers by the wrapped collector.
• Identity
  A subscriber that does nothing.
• Layered
  A collector composed of a collector wrapped by one or more subscribers.

Traits

• CollectExt
  Extension trait adding a with(Subscribe) combinator to types implementing Collect.
• Filterregistry and std
  A per-Subscribe filter that determines whether a span or event is enabled for an individual subscriber.
• Subscribe
  A composable handler for tracing events.