Expand description

Structured Logger

A logging implementation for the log crate that logs structured values either synchronous or asynchronous, in JSON, CBOR, or any other format, to a file, stderr, stdout, or any other destination. To initialize the logger use the Builder struct. It is inspired by std-logger.

This crate provides only a logging implementation. To do actual logging use the log crate and it’s various macros.

Limiting logging targets

You can use Builder::with_target_writer method to log messages related specific target to a specific writer.

Crate features

This crate has three features:

  • log-panic, enabled by default.

Log-panic feature

The log-panic feature will log all panics using the error severity, rather then using the default panic handler. It will log the panic message as well as the location and a backtrace, see the log output for an panic_log example.


Simple example:

use serde::Serialize;
use structured_logger::{async_json::new_writer, unix_ms, Builder};

async fn main() {
    // Initialize the logger.
        .with_target_writer("*", new_writer(tokio::io::stdout()))

    let kv = ContextLog {
        uid: "user123".to_string(),
        action: "upate_book".to_string(),

    log::info!("hello world");
    // This log will be written to stdout:
    // {"level":"INFO","message":"hello world","target":"simple","timestamp":1679745592127}

    log::info!(target: "api",
        method = "GET",
        path = "/hello",
        status = 200_u16,
        start = unix_ms(),
        elapsed = 10_u64,
        kv = log::as_serde!(kv);
    // This log will be written to stdout:
    // {"elapsed":10,"kv":{"uid":"user123","action":"upate_book"},"level":"INFO","message":"","method":"GET","path":"/hello","start":1679745592127,"status":200,"target":"api","timestamp":1679745592127}

struct ContextLog {
    uid: String,
    action: String,


  • Async JSON Writer Implementation
  • Sync JSON Writer Implementation


  • A struct to initialize the logger for log crate.


  • A trait that defines how to write a log. You can implement this trait for your custom formatting and writing destination.


  • Returns the log level from the environment variables: LOG, LOG_LEVEL, RUST_LOG, TRACE or DEBUG. Default is INFO.
  • A fallback logging function that is used in case of logging failure in Writer implementation. It will write failure information in JSON to stderr.
  • Returns the current unix timestamp in milliseconds.