slog 1.0.0-alpha3

Structured, composable logging for Rust

slog-rs - Structured, composable logging for Rust

Status & news

slog-rs is near final 1.0.0 release (see milestone 1.0).

Testing, feedback, PRs, etc. are very welcome. I'd be also very happy to share the ownership of the project with other people to make it more community-driven.

Long term goal is to make it a go-to logging crate for Rust.


  • easy to use
  • good performance; see: slog bench log
  • hierarchical loggers
  • lazily evaluated values
  • modular and extensible
    • small core create with multiple addon crates (./crates/) - compile only what you're actually using
  • backward compatibility with standard log crate (using slog-stdlog crate)
    • supports logging-scopes
  • drains & output formatting
    • filtering
      • compile time log level filter using cargo features (same as in log crate)
      • by level, msg, and other meta-data
      • slog-envlogger - port of env_logger
    • multiple outputs
    • asynchronous IO writing
    • 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 Rust trait system, 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 AsyncStreamer 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

Full mode:

slog-rs terminal full-format output

Compact mode:

slog-rs terminal compact output

Using & help

Code snippet

fn main() {
    // Create a new drain hierarchy, for the need of your program.
    // Choose from collection of existing drains, or write your own
    // `struct`-s implementing `Drain` trait.
    let drain = slog_term::streamer().async().full().build();

    // `AtomicSwitch` is a drain that wraps other drain and allows to change
    // it atomically in runtime.
    let ctrl = AtomicSwitchCtrl::new(drain);
    let drain = ctrl.drain();

    // Turn a drain into new group of loggers, sharing that drain.
    // Note `o!` macro for more natural `OwnedKeyValue` sequence building.
    let root = Logger::root(drain, o!("version" => VERSION, "build-id" => "8dfljdf"));

    // Build logging context as data becomes available.
    // Create child loggers from existing ones. Children clone `key: value`
    // pairs from their parents.
    let log =!("child" => 1));

    // Closures can be used for values that change at runtime.
    // Data captured by the closure needs to be `Send+Sync`.
    let counter = Arc::new(AtomicUsize::new(0));
    let log =!("counter" => {
        let counter = counter.clone();
        // Note the `move` to capture `counter`,
        // and unfortunate `|_ : &_|` that helps
        // current `rustc` limitations. In the future,
        // a `|_|` could work.
        move |_ : &Record| { counter.load(SeqCst)}

    // Loggers  can be cloned, passed between threads and stored without hassle.
    let join = thread::spawn({
        let log = log.clone();
        move || {

            info!(log, "before-fetch-add"); // counter == 0
            counter.fetch_add(1, SeqCst);
            info!(log, "after-fetch-add"); // counter == 1

            // `AtomicSwitch` drain can swap it's interior atomically (race-free).
                // drains are composable and reusable
                        // multiple outputs formats are supported

            // Closures can be used for lazy evaluation:
            // This `slow_fib` won't be evaluated, as the current drain discards
            // "trace" level logging records.
            debug!(log, "debug", "lazy-closure" => |_ : &Record| slow_fib(40));

            info!(log, "subthread", "stage" => "start");
            thread::sleep(Duration::new(1, 0));
            info!(log, "subthread", "stage" => "end");


See examples/ for full code.

Read Documentation for details and features.

See faq for answers to common questions. If you want to say hi, or need help use #slog-rs

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

Building & running

If you need to install Rust (come on, you should have done that long time ago!), use rustup.

In your project

In Cargo.toml:

slog = "*"

In your

extern crate slog;


Please fill an issue if slog does not fill your needs. I will appreciate any feedback. You might look into issue discussing slog-rs alternatives too.