slog 1.4.1

Structured, composable logging for Rust


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slog-rs - The Logging for Rust

Table of Contents

Status & news

slog is an ecosystem of reusable components for structured, composable logging for Rust.

The ambition is to be The Logging Framework for Rust. slog should accommodate variety of logging features and requirements.

slog crate


  • flexible & easy to use
  • great performance; see: slog bench log and what makes slog fast
  • #![no_std] support (with opt-out std cargo feature flag)
  • hierarchical loggers
  • lazily evaluated values
  • modular, lightweight and very extensible
    • tiny core crate that does not pull any dependencies
    • feature-crates for specific functionality
  • bidirectional compatibility with standard log crate (slog-stdlog crate)
    • supports logging-scopes
    • using slog in library does not force users of the library to use slog (but gives benefits); see crates/example-lib
  • drains & output formatting
    • filtering
      • compile-time log level filter using cargo features (same as in log crate)
      • by level, msg, and any other meta-data
      • slog-envlogger - port of env_logger
    • multiple outputs
    • efficient asynchronous logging
    • terminal output, with color support (slog-term crate)
    • Json (slog-json crate)
      • Bunyan (slog-bunyan crate)
    • syslog (slog-syslog crate)
    • first class custom drains

Advantages over log crate

  • extensible - slog provides core functionality, and some standard feature-set. But using traits, anyone can easily implement as powerful fully-custom features, publish separately and grow slog feature-set for everyone.
  • composable - Wouldn't it be nice if you could use env_logger, but output authentication messages to syslog, while reporting errors over network in json format? With slog drains can reuse other drains! You can combine them together, chain, wrap - you name it.
  • context aware - It's not just one global logger. Hierarchical loggers carry information about context of logging. When logging an error condition, you want to know which resource was being handled, on which instance of your service, using which source code build, talking with what peer, etc. In standard log you would have to repeat this information in every log statement. In slog it will happen automatically. See slog-rs functional overview page to understand better logger and drain hierarchies and log record flow through them.
  • both human and machine readable - By keeping the key-value data format, meaning of logging data is preserved. Dump your logging to a JSON file, and send it to your data-mining system for further analysis. Don't parse it from lines of text anymore!
  • lazy evaluation and asynchronous IO included. Waiting to finish writing logging information to disk, or spending time calculating data that will be thrown away at the current logging level, are sources of big performance waste. Use Async drain, and closures to make your logging fast.
  • run-time configuration - AtomicSwitch drain allows changing logging behavior in the running program. You could use eg. signal handlers to change logging level or logging destinations. See signal example.

Terminal output example

slog-term is only one of many slog features - useful showcase.

Automatic TTY detection and colors:

slog-rs terminal full-format output

Compact vs full mode:

slog-rs terminal compact output slog-rs terminal full output

Using & help

See examples/ for full quick code example overview.

See faq for answers to common questions and wiki for other documentation articles. If you want to say hi, or need help use slog-rs gitter channel.

Read Documentation for details and features.

To report a bug or ask for features use github issues.

In your project

In Cargo.toml:

slog = "1.2"

In your

extern crate slog;

Compatibility Policy

slog follows SemVer: this is the official policy regarding breaking changes and minimum required versions of Rust.

Slog crates should pin minimum required version of Rust to the CI builds. Bumping the minimum version of Rust is considered a minor breaking change, meaning at a minimum the minor version will be bumped.

In order to keep from being surprised of breaking changes, it is highly recommended to use the ~major.minor.patch style in your Cargo.toml if you wish to target a version of Rust that is older than current stable minus two releases:

slog = "~1.3.0"

This will cause only the patch version to be updated upon a cargo update call, and therefore cannot break due to new features, or bumped minimum versions of Rust.

Minimum Version of Rust

slog and it's ecosystem officially supports current stable Rust, minus two releases, but may work with prior releases as well. For example, current stable Rust at the time of this writing is 1.13.0, meaning slog is guaranteed to compile with 1.11.0 and beyond. At the 1.14.0 release, slog will be guaranteed to compile with 1.12.0 and beyond, etc.

Upon bumping the minimum version of Rust (assuming it's within the stable-2 range), it must be clearly annotated in the

Slog community


Slog related crates are hosted under slog github organization.

Dawid Ciężarkiewicz is the original author and current maintainer of slog and therefore self-appointed benevolent dictator over the project. When working on slog Dawid follows and expects everyone to follow his Code of Conduct.

Any particular repositories under slog ecosystem might be created, controlled, maintained by other entities with various level of autonomy. Lets work together toward a common goal in respectful and welcoming atmosphere!

slog-related resources