Axum Tracing and Structured Logging

Overview

Observability in an Axum service rests on two crates. tracing is the instrumentation facade: handler code and middleware emit events and create spans through it, with no knowledge of where the data goes. tracing-subscriber is the collector: it receives that data and decides what to filter, format, and export. The application wires them together exactly once, in main, before the server starts.

A subscriber is built compositionally: a Registry for span storage plus one or more Layers, each adding one behavior (filtering, console formatting, OTLP export). The tracing and tracing-subscriber APIs in this skill are identical for Axum 0.7 and 0.8; the version difference in Axum does not reach this layer.

Three rules dominate correct tracing code: