sjl - Simple JSON Logger

📦 crates.io | 📚 docs.rs

What

It's a Simple JSON Logger. It logs JSON to stderr.

Why?

The most popular logging crate, tracing, has problems with nested JSON unless you use the valuable crate with it which is unstable and behind a feature flag for 3 years... but that still has issues with enums and doesn't feel natural to use with .as_value() everywhere. The slog crate has similar issues—I've written about both here.

If you just want a Simple JSON Logger, you might find this useful.

Installation

cargo add sjl

Usage

use sjl::Logger;

fn main() {
    let logger = Logger::new();

    logger.info("Saul Goodman", ()); // 2nd param is optional data
}

Outputs

{"timestamp":"2026-05-21T02:45:03.456Z","level":"info","message":"Saul Goodman"}

Extended Usage / Raison d'être

use serde::Serialize;
use sjl::LoggerOptions;

#[derive(Serialize)] // <-- All you need!
struct User {
    name: String,
    cars: Vec<Car>,
}

#[derive(Serialize)]
struct Car {
    make: String,
    model: String,
    transmission: Transmission,
}

#[derive(Serialize)]
enum Transmission {
    Automatic,
    Manual,
}

fn main() {
    let logger = LoggerOptions::default().pretty(true).init();

    let user = User {
        name: "Jose".into(),
        cars: vec![
            Car {
                make: "Toyota".into(),
                model: "Rav4".into(),
                transmission: Transmission::Manual,
            },
            Car {
                make: "Tesla".into(),
                model: "Cybertruck".into(),
                transmission: Transmission::Automatic,
            },
        ],
    };

    logger.info("Saul Goodman!", &user);
}

Outputs

{
  "timestamp": "2026-05-21T03:39:36.780Z",
  "level": "info",
  "message": "Saul Goodman!",
  "data": {
    "name": "Jose",
    "cars": [
    // No escaped strings!
      {
        "make": "Toyota",
        "model": "Rav4",
        "transmission": "Manual" // Enums render normally
      },
      {
        "make": "Tesla",
        "model": "Cybertruck",
        "transmission": "Automatic"
      }
    ]
  }
}

All Options

use std::time::Duration;
use sjl::{LoggerOptions, LogLevel};

fn main() {
    let logger = LoggerOptions::default()
        // Context are k/v pairs that are added to every log line
        // use these for identifiers like service, environment, version, etc.
        .context("service", "payments")
        .context("environment", "production")
        // Minimum severity that actually gets emitted.
        // For example, setting this to Info will not show Debug logs
        // Hierarchy: Debug < Info < Warn < Error
        .min_level(LogLevel::Warn)
        // Batching
        // Flush once the batch reaches this many bytes
        .flush_at_bytes(1_000)
        // ...or once we have this many messages
        .flush_at_messages(100)
        // ...or once this much time has passed since the last flush.
        // Whatever comes first wins.
        .flush_interval(Duration::from_millis(250))
        // Buffer pool
        // How many buffers to keep in the pool
        // Set this to around your expected concurrent in-flight log count
        .buffer_pool_size(20)
        // Starting capacity (in bytes) of each buffer. Tune this to your typical log size
        // so that hot path logging never has to grow the buffers
        .buffer_pool_initial_capacity(4_000)
        // Hard cap on how big the buffers can get. Any that exceed this size
        // will get shrunk back down before being returned to the pool
        // So that one giant log can't be a memory hog.
        // Oversized logs also trigger occasional warnings
        .buffer_pool_max_capacity(100_000)
        // Rename the `timestamp` field in the output
        .timestamp_key("time")
        // Custom chrono strftime format. Default is RFC 3339 with milliseconds.
        // Build your own from here: https://docs.rs/chrono/latest/chrono/format/strftime/index.html
        .timestamp_format("%FT%I:%M:%S%p")
        // Pretty-print JSON using multiple lines. Default is compact, single line.
        .pretty(true)
        // Spawns a background worker thread and returns the logger
        .init();

    logger.error("Saul Goodman!", ());

}

Outputs

{
  "time": "2026-05-21T03:35:04AM",
  "level": "error",
  "message": "Saul Goodman!",
  "environment": "production",
  "service": "payments"
}

Running Tests

cargo llvm-cov --html