Logging

Logging is the process of recording significant events, actions, or errors within a software system. Typically, it involves recording them in a textual format as log messages, with the ability to designate each at different levels (such as error, warning, info, debug). This can be used to observe a system (such as flagging error logs) or to debug issues (such as deducing why a system is failing from debug or info logs).

There are some additional ways that logging can be implemented:

• Structured logging involves adding metadata to log messages, often in the form of key-value pairs. This data can be used to filter log messages, for example by user or by resource.
• Tracing means generating log events to be able to trace the propagation of asynchronous tasks. For example, it might mean that the log library issues log events whenever an asynchronous task (request handler) is launched, and when it is completed, or attach metadata to a log event to be able to trace its propagation through several services.

The Rust ecosystem has centred around three crates which are used for logging. These vary in terms of their intended use-case, and to some extend can even be mixed through interop libraries. In the next sections, we will discuss each of them and finally show some ways to mix-and-match them.

For your Rust project, it makes sense to think about what you want our of your logging system and choose the right kind of logging infrastructure. Part of the consideration should also be what other libraries or frameworks you are using, because many of them come with logging support built-in that you can enable. For example, many of the asynchronous HTTP libraries have built-in support for the tracing crate.

Log

The log crate is the most popular logging infrastructure.

Tracing

The tracing crate implements scoped structured logging. It is maintained by the Tokio developers and is commonly used in async projects, as it excels at tracing asynchronous functions.

Slog

slog is a logging crate for Rust with the tagline structured, contextual, extensible, composable logging for Rust.

Interopability

Reading

Structured logging By Rust telemetry exercises

What is the Difference Between Tracing and Logging? by Amanda Viescinski