captains-log

A light-weight customizable logger implementation for rust

crates.io: docs.rs:

Features

• Allow customize log format and time format. Refer to LogFormat

• Supports multiple types of sink stacking, each with its own log level.

  • LogConsole: Console output to stdout/stderr.

  • LogRawFile: Support atomic appending from multi-process on linux

  • LogBufFile : Write to log file with merged I/O and delay flush, and optional self-rotation.

  • Syslog: (feature syslog)

    Write to local or remote syslog server, with timeout and auto reconnect.

  • LogRingFile: (feature ringfile)

    For deadlock / race condition debugging, collect log to ring buffer in memory.

• Log panic message by default.

• Provide additional macros. For example: log_assert!(), logger_assert!() ...

• Supports signal listening for log-rotate. Refer to Builder::signal()

• Provides many preset recipes in [recipe] module for convenience.

• Supports configured by environment.

• Fine-grain module-level control.

  Provides LogFilter to filter specified logs on-the-fly.

• API-level log handling.

  Provides LogFilterKV for API logging with additional key.

  For example, you can set req_id in LogFilterKV, and track the complete request handling procedure from log.

• For test suits usage:

  • Allow dynamic reconfigure logger setting in different test function.

    Refer to Unit test example.

  • Provides an attribute macro #[logfn] to wrap test function.

    Refer to Best practice with rstest

• Provides a parser to work on your log files.

Usage

Cargo.toml

[dependencies]
log = { version = "0.4", features = ["std", "kv_unstable"] }
captains_log = "0.8"

lib.rs or main.rs:

// By default, reexport the macros from log crate
#[macro_use]
extern crate captains_log;

Fast setup example

You can refer to various preset recipe in recipe module.

The following is setup two log files for different log-level:

#[macro_use]
extern crate captains_log;
use captains_log::recipe;

// You'll get /tmp/test.log with all logs, and /tmp/test.log.wf only with error logs.
let mut log_builder = recipe::split_error_file_logger("/tmp", "test", log::Level::Debug);
// Builder::build() is equivalent of setup_log()
log_builder.build();

// non-error msg will only appear in /tmp/test.log
debug!("Set a course to Sol system");
info!("Engage");
// will appear in both /tmp/test.log and /tmp/test.log.wf
error!("Engine over heat!");

Buffered sink with log rotation (See the definition of Rotation):

#[macro_use]
extern crate captains_log;
use captains_log::*;
// rotate when log file reaches 512M. Keep max 10 archiveed files, with recent 2 not compressed.
// All archived log is moved to "/tmp/rotation/old"
let rotation = Rotation::by_size(1024 * 4 * 2, max_files)
        .compress_exclude(2).archive_dir("/tmp/rotation/old");
let _ = recipe::buffered_rotated_file_logger("/tmp/rotation.log", Level::Debug, rotation).build();

Configure by environment

There is a recipe env_logger() to configure a file logger or console logger from env. As simple as:

use captains_log::recipe;
let _ = recipe::env_logger("LOG_FILE", "LOG_LEVEL").build();

If you want to custom more, setup your config with env_or() helper.

Customize format example

use captains_log::*;

fn format_f(r: FormatRecord) -> String {
    let time = r.time();
    let level = r.level();
    let file = r.file();
    let line = r.line();
    let msg = r.msg();
    format!("{time}|{level}|{file}:{line}|{msg}\n").to_string()
}
let debug_format = LogFormat::new(
    "%Y%m%d %H:%M:%S%.6f",
    format_f,
);
let debug_file = LogRawFile::new(
    "/tmp", "test.log", log::Level::Trace, debug_format);
let config = Builder::default()
    .signal(signal_hook::consts::SIGINT)
    .add_sink(debug_file);

config.build();

Fine-grain module-level control

Place LogFilter in Arc and share among coroutines. Log level can be changed on-the-fly.

There're a set of macro "logger_XXX" to work with LogFilter.

use std::sync::Arc;
use captains_log::*;
log::set_max_level(log::LevelFilter::Debug);
let logger_io = Arc::new(LogFilter::new());
let logger_req = Arc::new(LogFilter::new());
logger_io.set_level(log::Level::Error);
logger_req.set_level(log::Level::Debug);
logger_debug!(logger_req, "Begin handle req ...");
logger_debug!(logger_io, "Issue io to disk ...");
logger_error!(logger_req, "Req invalid ...");

API-level log handling

Request log can be track by customizable key (for example, "req_id"), which kept in [LogFilterKV], and LogFilterKV is inherit from LogFilter.

You need macro "logger_XXX" to work with it.

use captains_log::*;
fn debug_format_req_id_f(r: FormatRecord) -> String {
    let time = r.time();
    let level = r.level();
    let file = r.file();
    let line = r.line();
    let msg = r.msg();
    let req_id = r.key("req_id");
    format!("[{time}][{level}][{file}:{line}] {msg}{req_id}\n").to_string()
}
let builder = recipe::raw_file_logger_custom("/tmp", "log_filter", log::Level::Debug,
    recipe::DEFAULT_TIME, debug_format_req_id_f);
builder.build().expect("setup_log");
let logger = LogFilterKV::new("req_id", format!("{:016x}", 123).to_string());
logger_debug!(logger, "Req / received");
logger_debug!(logger, "header xxx");
logger_info!(logger, "Req / 200 complete");

The log will be:

[2025-06-11 14:33:08.089090][DEBUG][request.rs:67] API service started
[2025-06-11 14:33:10.099092][DEBUG][request.rs:67] Req / received (000000000000007b)
[2025-06-11 14:33:10.099232][WARN][request.rs:68] header xxx (000000000000007b)
[2025-06-11 14:33:11.009092][DEBUG][request.rs:67] Req / 200 complete (000000000000007b)

Unit test example

To setup different log config on different tests.

Make sure that you call Builder::test() in test cases. which enable dynamic log config and disable signal_hook.

use captains_log::*;

#[test]
fn test1() {
    recipe::raw_file_logger(
        "/tmp", "test1", Level::Debug).test().build();
    info!("doing test1");
}

#[test]
fn test2() {
    recipe::raw_file_logger(
        "/tmp", "test2", Level::Debug).test().build();
    info!("doing test2");
}

Best practice with rstest

We provides proc macro [logfn], the following example shows how to combine with rstest.

• When you have large test suit, you want to know which logs belong to which test case.

• Sometimes your test crashes, you want to find the responsible test case.

• The time spend in each test.

use rstest::*;
use log::*;
use captains_log::*;

// A show case that setup() fixture will be called twice, before each test.
// In order make logs available.
#[logfn]
#[fixture]
fn setup() {
    let builder = recipe::raw_file_logger("/tmp", "log_rstest", log::Level::Debug).test();
    builder.build().expect("setup_log");
}

#[logfn]
#[rstest(file_size, case(0), case(1))]
fn test_rstest_foo(setup: (), file_size: usize) {
    info!("do something111");
}

#[logfn]
#[rstest]
fn test_rstest_bar(setup: ()) {
    info!("do something222");
}

After running the test with: cargo test -- --test-threads=1

/tmp/log_rstest.log will have this content:

[2025-06-21 00:39:37.091326][INFO][test_rstest.rs:11] >>> setup return () >>>
[2025-06-21 00:39:37.091462][INFO][test_rstest.rs:27] <<< test_rstest_bar (setup = ()) enter <<<
[2025-06-21 00:39:37.091493][INFO][test_rstest.rs:30] do something222
[2025-06-21 00:39:37.091515][INFO][test_rstest.rs:27] >>> test_rstest_bar return () >>>
[2025-06-21 00:39:37.091719][INFO][test_rstest.rs:11] <<< setup () enter <<<
[2025-06-21 00:39:37.091826][INFO][test_rstest.rs:11] >>> setup return () >>>
[2025-06-21 00:39:37.091844][INFO][test_rstest.rs:21] <<< test_rstest_foo (setup = (), file_size = 0) enter <<<
[2025-06-21 00:39:37.091857][INFO][test_rstest.rs:24] do something111
[2025-06-21 00:39:37.091868][INFO][test_rstest.rs:21] >>> test_rstest_foo return () >>>
[2025-06-21 00:39:37.092063][INFO][test_rstest.rs:11] <<< setup () enter <<<
[2025-06-21 00:39:37.092136][INFO][test_rstest.rs:11] >>> setup return () >>>
[2025-06-21 00:39:37.092151][INFO][test_rstest.rs:21] <<< test_rstest_foo (setup = (), file_size = 1) enter <<<
[2025-06-21 00:39:37.092163][INFO][test_rstest.rs:24] do something111
[2025-06-21 00:39:37.092173][INFO][test_rstest.rs:21] >>> test_rstest_foo return () >>>