Crate tracing_appender

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Writers for logging events and spans


tracing is a framework for 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:

File Appender

The rolling module provides functions to create rolling and non-rolling file appenders.

Rolling file appender rotation options are Rotation::MINUTELY, Rotation::HOURLY, and Rotation::DAILY.

To create a non-rolling file appender, use tracing_appender::rolling::never(/*...*/) or Rotation::NEVER.

The following example creates an hourly rotating file appender that writes to /some/directory/prefix.log.YYYY-MM-DD-HH:

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

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.

See the rolling module’s documentation for more detail on how to use this file appender.

Non-Blocking Writer

The example below demonstrates the construction of a non_blocking writer with std::io::stdout(), which implements MakeWriter.

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

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:

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<()> {

let (non_blocking, _guard) = tracing_appender::non_blocking(TestWriter);

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

Non-Blocking Rolling File Appender

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.



  • Convenience function for creating a non-blocking, off-thread writer.