Day 4 - slog

Structured logging

I hope you're using any sort of logging in your applications. Even if that means printing stuff to stdout and relying on shell output redirection, it's still better than no logging at all. However, most programming languages have great libraries for logging. Be it Java, Python, Elixir - there are logging utilities everywhere. But more often than not, what is logged is some sort of prose that a programmer thought was applicable at a time.

I once wrote this line of Python code:

logger.error('Oh no, something terrible happened: %s', details)

Even though I've configured my logging levels, formatters and other stuff that's very useful, in the end I still had to grep terrible to find what actually happened, and later parse the details with a nasty regexp. Not very convenient. Structured logging is a concept that puts events over messages; we're now logging events with associated key-value data, not plain string messages.

slog basics

slog is the single most popular crate for structured logging in Rust. It supports various outputs (called drains in slog terminology), but for now we'll stick to the terminal output provided by the slog_term crate.

extern crate slog;
extern crate slog_term;

use slog::DrainExt;

fn main() {
    let drain = slog_term::streamer().build().fuse();
    let root_logger = slog::Logger::root(drain, o!("version" => "0.5"));
    info!(root_logger, "Application started";
        "started_at" => format!("{}", time::now().rfc3339()));

The first line in main() creates and configures a drain which will output log messages to the terminal. By default, slog_term autodetects color terminals and uses a compact style for output. Next we're creating the root logger. Every application must have a root logger, so it makes sense to provide some initial logging context from the start, such as app version. We're using a convenience 'o!` macro to define that context as key-value pairs.

Once we have a logger, we can log stuff! There's a couple macros such as debug!, info!, err! etc. to log events of different severity. We can add more context in each call to these macros. Here lies the power of structured logging: it's composable. Every time we add some more context, we retain the context defined when logger was created. See the output:

$ cargo run
Nov 30 20:46:47.073 INFO Application started, version: 0.5, started_at: 2016-11-30T20:46:47+01:00

Even though the call to info! doesn't mention app version at all, it is there. Cool, but there's more. We can create child loggers that will inherit their context from parent logger. Let's assume our app has some users that can sign in.

struct User {
    username: String,
    logger: slog::Logger,

impl User {
    fn new(username: &str, logger: &slog::Logger) -> Self {
        User {
            username: username.to_string(),
            logger:!("username" => username.to_string())),

    fn sign_in(&self) {
        info!(self.logger, "User signed in");

// in main:
let user = User::new("zbyszek", &root_logger);
$ cargo run
Nov 30 20:59:42.811 INFO User signed in, username: zbyszek, version: 0.5

Our User objects contain their own loggers that inherit from the root logger and add context relevant to user interactions. The event inside sign_in() method still logs app version coming from the root logger, while enhancing the context with the username of the user who signs in. Again, composition is a strong selling point of slog.

Note: we can avoid passing Logger around in the arguments, thanks to the slog-scope crate. I decided not to do that here to make the examples more explicit.

Other outputs

With slog-stream we can have drains that write events to anything implementing io::Write. Let's build a drain that writes both to stdout and a log file.

extern crate slog_json;
extern crate slog_stream;

use std::fs::File;

let console_drain = slog_term::streamer().build();
let file = File::create("app.log").expect("Couldn't open log file");
let file_drain = slog_stream::stream(file, slog_json::default());
let logger = slog::Logger::root(slog::duplicate(console_drain, file_drain).fuse(), o!());
warn!(logger, "not_enough_resources"; "resource" => "cat pictures");

The slog_json::default() function creates a JSON formatter with some convenient keys like timestamp and log level already added to the objects. We feed it in the second argument to stream() which accepts anything that is writable - like files, stdout or network sockets. Later we use duplicate() to create a higher level drain that will forward events to both outputs.

$ cargo run
Dec 01 22:25:08.520 WARN not_enough_resources, resource: cat pictures
$ cat app.log
{"ts":"2016-12-01T22:25:08.521265800+01:00","level":"WARN","msg":"not_enough_resources","resource":"cat pictures"}

Of course we could stream events into something more appropriate than a simple file, like Logstash or Graylog.

There's a few more bits and pieces related to slog. We can filter log levels either with drains, or entirely at compile time. slog itself removes trace and debug logging in release builds. There's also a way to log events asynchronously to improve performance. Furthermore, if you've already used env_logger before, slog comes with a shim to ease transition.

Further reading