sjl - Simple JSON Logger

📦 crates.io | 📚 docs.rs

Why?

I mostly need JSON logging without the quirks: enums that serialize correctly and clean output out of the box, not escaped strings.

I built this because the tracing crate's valuable support has been behind an unstable feature flag for over three years and the slog crate also doesn't seem to provide this..

If you want a simple JSON logger, this might be useful for you too.

Installation

cargo add sjl

Usage

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

#[derive(Serialize)]
struct User {
   id: u64,
   name: String,
   verified: bool,
}

#[derive(Serialize)]
enum Status {
   Active,
   Inactive,
   RateLimited { retry_after: u32 },
}

#[derive(Serialize)]
struct Address {
   street: String,
   city: String,
   country: String,
}

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

#[derive(Serialize)]
struct Order {
   order_id: String,
   user: User,
   status: Status,
   shipping_address: Address,
   items: Vec<OrderItem>,
   metadata: Metadata,
}

#[derive(Serialize)]
struct Metadata {
   tags: Vec<String>,
   priority: u8,
   notes: Option<String>,
}

fn main() {
   // Initialize once at startup
   Logger::init()
       .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)
       .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))
       .build();

   // 1. Simple string messages
   debug!("Application started");

   // 2. String with message context
   info!("Server listening", "0.0.0.0:8080");

   // 3. Struct data (works seamlessly)
   info!(User {
       id: 1,
       name: "Alice".into(),
       verified: true
   });

   // 4. Message + struct data
   info!("User authenticated", User {
       id: 1,
       name: "Alice".into(),
       verified: true
   });

   // 5. Enum variants serialize correctly
   warn!(Status::Active);
   warn!(Status::RateLimited { retry_after: 60 });

   // 6. Ad-hoc JSON without defining structs
   error!(json!({
       "error": "connection_failed",
       "host": "db.example.com",
       "port": 5432
   }));

   // 7. Message + ad-hoc JSON
   error!("Database connection failed", json!({
       "host": "db.example.com",
       "port": 5432,
       "retry_count": 3
   }));

   // 8. Complex nested: Vec of structs containing enums
   info!(
       "Order processed",
       Order {
           order_id: "ORD-2024-001".into(),
           user: User {
               id: 42,
               name: "John Doe".into(),
               verified: true,
           },
           status: Status::Active,
           shipping_address: Address {
               street: "123 Main St".into(),
               city: "San Francisco".into(),
               country: "USA".into(),
           },
           items: vec![
               OrderItem {
                   sku: "WIDGET-001".into(),
                   name: "Premium Widget".into(),
                   quantity: 2,
                   price: 29.99,
                   status: Status::Active,
               },
               OrderItem {
                   sku: "GADGET-002".into(),
                   name: "Super Gadget".into(),
                   quantity: 1,
                   price: 49.99,
                   status: Status::RateLimited { retry_after: 30 },
               },
           ],
           metadata: Metadata {
               tags: vec!["express".into(), "gift".into()],
               priority: 1,
               notes: Some("Handle with care".into()),
           },
       }
   );
}

Output

{"level":"DEBUG","timestamp":"2025-10-28T10:14:41.229Z","data":"Application started"}
{"level":"INFO","timestamp":"2025-10-28T10:14:41.229Z", "message": "Server listening","data":"0.0.0.0:8080"}
{"level":"INFO","timestamp":"2025-10-28T10:14:41.229Z","data":{"id":1,"name":"Alice","verified":true}}
{"level":"INFO","timestamp":"2025-10-28T10:14:41.229Z", "message": "User authenticated","data":{"id":1,"name":"Alice","verified":true}}
{"level":"WARN","timestamp":"2025-10-28T10:14:41.229Z","data":"Active"}
{"level":"WARN","timestamp":"2025-10-28T10:14:41.229Z","data":{"RateLimited":{"retry_after":60}}}
{"level":"ERROR","timestamp":"2025-10-28T10:14:41.229Z","data":{"error":"connection_failed","host":"db.example.com","port":5432}}
{"level":"ERROR","timestamp":"2025-10-28T10:14:41.229Z", "message": "Database connection failed","data":{"host":"db.example.com","port":5432,"retry_count":3}}
{"level":"INFO","timestamp":"2025-10-28T10:14:41.229Z", "message": "Order processed","data":{"items":[{"name":"Premium Widget","price":29.99,"quantity":2,"sku":"WIDGET-001","status":"Active"},{"name":"Super Gadget","price":49.99,"quantity":1,"sku":"GADGET-002","status":{"RateLimited":{"retry_after":30}}}],"metadata":{"notes":"Handle with care","priority":1,"tags":["express","gift"]},"order_id":"ORD-2024-001","shipping_address":{"city":"San Francisco","country":"USA","street":"123 Main St"},"status":"Active","user":{"id":42,"name":"John Doe","verified":true}}}