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MIT license

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Tracing — Structured, application-level diagnostics


Writers for logging events and spans

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tracing is a framework for instrumenting Rust programs to collect structured, event-based diagnostic information. tracing-appender allows events and spans to be recorded in a non-blocking manner through a dedicated logging thread. It also provides a RollingFileAppender that can be used with or without the non-blocking writer.

Compiler support: requires rustc 1.63+


Add the following to your Cargo.toml:

tracing-appender = "0.2"

This crate can be used in a few ways to record spans/events:

Rolling File Appender

fn main(){
    let file_appender = tracing_appender::rolling::hourly("/some/directory", "prefix.log");

This creates an hourly rotating file appender that writes to /some/directory/prefix.log.YYYY-MM-DD-HH. Rotation::DAILY and Rotation::NEVER are the other available options.

The file appender implements std::io::Write. To be used with tracing_subscriber::FmtSubscriber, it must be combined with a MakeWriter implementation to be able to record tracing spans/event.

The rolling module's documentation provides more detail on how to use this file appender.

Non-Blocking Writer

The example below demonstrates the construction of a non_blocking writer with an implementation of std::io::Writer.

use std::io::Error;

struct TestWriter;

impl std::io::Write for TestWriter {
    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
        let buf_len = buf.len();
        println!("{:?}", buf);

    fn flush(&mut self) -> std::io::Result<()> {

fn main() {
    let (non_blocking, _guard) = tracing_appender::non_blocking(TestWriter);

Note: _guard is a WorkerGuard which is returned by tracing_appender::non_blocking to ensure buffered logs are flushed to their output in the case of abrupt terminations of a process. See WorkerGuard module for more details.

The example below demonstrates the construction of a tracing_appender::non_blocking writer constructed with a std::io::Write:

fn main() {
    let (non_blocking, _guard) = tracing_appender::non_blocking(std::io::stdout());

The non_blocking module's documentation provides more detail on how to use non_blocking.

Non-Blocking Rolling File Appender

fn main() {
    let file_appender = tracing_appender::rolling::hourly("/some/directory", "prefix.log");
    let (non_blocking, _guard) = tracing_appender::non_blocking(file_appender);

Supported Rust Versions

tracing-appender is built against the latest stable release. The minimum supported version is 1.63. The current tracing-appender version is not guaranteed to build on Rust versions earlier than the minimum supported version.

Tracing follows the same compiler support policies as the rest of the Tokio project. The current stable Rust compiler and the three most recent minor versions before it will always be supported. For example, if the current stable compiler version is 1.69, the minimum supported version will not be increased past 1.66, three minor versions prior. Increasing the minimum supported compiler version is not considered a semver breaking change as long as doing so complies with this policy.


This project is licensed under the MIT license.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Tokio by you, shall be licensed as MIT, without any additional terms or conditions.


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