Crate tracing_subscriber[][src]

Expand description

Utilities for implementing and composing tracing subscribers.

tracing is a framework for instrumenting Rust programs to collect scoped, structured, and async-aware diagnostics. The Subscriber trait represents the functionality necessary to collect this trace data. This crate contains tools for composing subscribers out of smaller units of behaviour, and batteries-included implementations of common subscriber functionality.

tracing-subscriber is intended for use by both Subscriber authors and application authors using tracing to instrument their applications.

Compiler support: requires rustc 1.42+

Layers and Filters

The most important component of the tracing-subscriber API is the Layer trait, which provides a composable abstraction for building Subscribers. Like the Subscriber trait, a Layer defines a particular behavior for collecting trace data. Unlike Subscribers, which implement a complete strategy for how trace data is collected, Layers provide modular implementations of specific behaviors. Therefore, they can be composed together to form a Subscriber which is capable of recording traces in a variety of ways. See the layer module’s documentation for details on using Layers.

In addition, the Filter trait defines an interface for filtering what spans and events are recorded by a particular layer. This allows different Layers to handle separate subsets of the trace data emitted by a program. See the documentation on per-layer filtering for more information on using Filters.

Included Subscribers

The following Subscribers are provided for application authors:

  • fmt - Formats and logs tracing data (requires the fmt feature flag)

Feature Flags

  • env-filter: Enables the EnvFilter type, which implements filtering similar to the env_logger crate. Enabled by default.
  • fmt: Enables the fmt module, which provides a subscriber implementation for printing formatted representations of trace events. Enabled by default.
  • ansi: Enables fmt support for ANSI terminal colors. Enabled by default.
  • registry: enables the registry module. Enabled by default.
  • json: Enables fmt support for JSON output. In JSON output, the ANSI feature does nothing.

Optional Dependencies

  • tracing-log: Enables better formatting for events emitted by log macros in the fmt subscriber. On by default.
  • chrono: Enables human-readable time formatting in the fmt subscriber. Enabled by default.
  • smallvec: Causes the EnvFilter type to use the smallvec crate (rather than Vec) as a performance optimization. Enabled by default.
  • parking_lot: Use the parking_lot crate’s RwLock implementation rather than the Rust standard library’s implementation.

Supported Rust Versions

Tracing is built against the latest stable release. The minimum supported version is 1.42. The current Tracing version is not guaranteed to build on Rust versions earlier than the minimum supported version.

Tracing follows the same compiler support policies as the rest of the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.45, the minimum supported version will not be increased past 1.42, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.


pub use layer::Layer;
pub use fmt::Subscriber as FmtSubscriber;
pub use fmt::fmt;


Utilities for working with fields and field visitors.

Layers that control which spans and events are enabled by the wrapped subscriber.


A Subscriber for formatting and logging tracing data.

The Layer trait, a composable abstraction for building Subscribers.

The tracing-subscriber prelude.

Storage for span data shared by multiple Layers.

Wrapper for a Layer to allow it to be dynamically reloaded.

Extension traits and other utilities to make working with subscribers more ergonomic.



Tracks the currently executing span on a per-thread basis.


A Layer which filters spans and events based on a set of filter directives.


A shared, reusable store for spans.