GitHub - joswayski/sjl: Simple JSON Logger with proper enum and nested JSON support (no escaped strings!)

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.

Features

Batched, non-blocking writes
Graceful shutdown (flushes on exit)
Falls back to sync writes if buffer is full
Customizable colors, timestamps, batch sizes
Pretty-printing mode for development
Works with any Serialize type
Macros! debug!(), info!(), warn!(), and error!()
Global context fields that appear in every log message

Installation

Usage

use sjl::{debug, error, info, warn, LogLevel, Logger, RGB};
use serde::Serialize;
use serde_json::json;

#[derive(Serialize)] // This is all you need
struct User {
    id: u64,
    name: String,
}

#[derive(Serialize)]
enum OrderStatus {
    Pending,
    Shipped { tracking_number: String },
    Delivered,
}

#[derive(Serialize)]
struct Order {
    user: User,
    items: Vec<OrderItem>,
}

#[derive(Serialize)]
struct OrderItem {
    name: String,
    price: f64,
    quantity: u32,
    status: OrderStatus,
}

fn main() {
    // Initialize once at startup
    Logger::init()
        // Optional config
        .min_level(LogLevel::Debug)       // Minimum log level (default: Debug)
        .batch_size(100)                  // Logs per batch (default: 50)
        .batch_duration_ms(100)           // Max ms before flush (default: 50)
        .buffer_size(5000)                // Channel capacity (default: 1024)
        .timestamp_format("%Y-%m-%dT%H:%M:%S%.3fZ")  // ISO 8601 (default)
        .timestamp_key("tz")              // Rename this field if you want (default: timestamp)
        .pretty(true)                     // Pretty-print JSON (default: false)
        .debug_color(RGB::new(38, 45, 56))   // Customize colors
        .info_color(RGB::new(15, 115, 255))
        .warn_color(RGB::new(247, 155, 35))
        .error_color(RGB::new(255, 0, 0))
        // Context fields appear in EVERY log message at the top level
        .context("environment", "production")
        .context("service", "order-api")
        .context("metadata", json!({
            "instance_id": "i-1234567890abcdef0",
            "pod_name": "order-api-7d4f8c9b5-x8k2p",
            "git_sha": "abc123f"
        }))
        // Call this at the end
        .build(); 

    // Strings
    debug!("App started");
    info!("Server listening", "0.0.0.0:8080");

    // Structs
    info!(User { id: 1, name: "Alice".into() });
    info!("User authenticated", User { id: 1, name: "Alice".into() });

    // Enums (serialize correctly!)
    warn!(OrderStatus::Pending);
    warn!(OrderStatus::Shipped { tracking_number: "1Z999AA10123456784".into() });

    // Ad-hoc JSON
    error!(json!({
        "error": "connection_failed",
        "host": "db.example.com"
    }));

    // Complex: Vec of structs with enums
    info!("Order processed", Order {
        user: User { id: 42, name: "John".into() },
        items: vec![
            OrderItem {
                name: "Widget".into(),
                price: 29.99,
                quantity: 2,
                status: OrderStatus::Shipped { tracking_number: "1Z999AA10123456784".into() },
            },
            OrderItem {
                name: "Gadget".into(),
                price: 49.99,
                quantity: 1,
                status: OrderStatus::Pending,
            },
        ],
    });
}

Pretty Mode

When .pretty(true) is enabled, logs are formatted with indentation and newlines for easier reading during development:

{
  "data": {
    "items": [
      {
        "name": "Widget",
        "price": 29.99,
        "quantity": 2,
        "status": {
          "Shipped": {
            "tracking_number": "1Z999AA10123456784"
          }
        }
      },
      {
        "name": "Gadget",
        "price": 49.99,
        "quantity": 1,
        "status": "Pending"
      }
    ],
    "user": {
      "id": 42,
      "name": "John"
    }
  },
  "environment": "production",
  "level": "INFO",
  "message": "Order processed",
  "metadata": {
    "git_sha": "abc123f",
    "instance_id": "i-1234567890abcdef0",
    "pod_name": "order-api-7d4f8c9b5-x8k2p"
  },
  "service": "order-api",
  "tz": "2025-10-31T07:33:05.627Z"
}

Compact Mode (Default)

With .pretty(false) or omitted (default), logs are output as single-line JSON:

{"level":"DEBUG","tz":"2025-10-31T07:33:44.333Z","message":"App started","environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"INFO","tz":"2025-10-31T07:33:44.333Z","message":"Server listening","data":"0.0.0.0:8080","environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"INFO","tz":"2025-10-31T07:33:44.333Z","data":{"id":1,"name":"Alice"},"environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"INFO","tz":"2025-10-31T07:33:44.333Z","message":"User authenticated","data":{"id":1,"name":"Alice"},"environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"WARN","tz":"2025-10-31T07:33:44.333Z","message":"Pending","environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"WARN","tz":"2025-10-31T07:33:44.333Z","data":{"Shipped":{"tracking_number":"1Z999AA10123456784"}},"environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}
{"level":"ERROR","tz":"2025-10-31T07:33:44.333Z","data":{"error":"connection_failed","host":"db.example.com"},"environment": "production", "service": "order-api", "metadata": {"git_sha":"abc123f","instance_id":"i-1234567890abcdef0","pod_name":"order-api-7d4f8c9b5-x8k2p"}}