slog 0.7.0

Structured, composable logging for Rust

slog-rs - Structured, composable logging for Rust


Inspired by log15 for Go.


  • easy to use
  • hierarchical loggers
  • lazily evaluated values
  • modular and extensible
    • small core create with multiple addon crates (./crates/) - compile only what you're actually using
  • standard log crate logging support (slog-stdlog crate)
  • drains & output formatting
    • filtering
    • 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)
    • support for first class custom ones

Advantages over log crate

  • composable and extensible logging drains - You can easily log messages to different destinations, in different formats, using different filtering rules. Building new drains and new output formats is very easy.
  • drains are run-time swappable - Send a signal to your program and start logging everything to a file for run-time production debugging. Send another one when you're done to return to normal operation. Build your own flexible scenarios easily.
  • non-global - 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.
  • 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.
  • lazy evaluation and asynchronous IO included

Terminal output example

slog-rs terminal 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::async_stderr();

    // `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 = drain.into_logger(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 |_ : &RecordInfo| { 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" => |_ : &RecordInfo| 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.

If you want to say hi, or need help use #dpc

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;