Recipe

Instrument a request handler

Use when: a service handles many requests at once and its log lines can't be told apart or searched

Goal: every line a request writes carries that request's facts as fields, at the right level, with nothing private in it

Taught in Structured logging and tracing

1. 1

   Call tracing_subscriber::fmt() with an EnvFilter, and .json() when the output goes to a log store.

   Why: the macros record nothing until a subscriber exists, and one filter controls every crate's output.

2. 2

   Add #[instrument(skip_all, fields(...))] or skip(...) for anything private or large.

   Example

   #[instrument(skip(state, req), fields(sailing = %req.sailing))] on async fn book(...).

   Why: the span stamps every event inside the call, including those from functions it calls, with the request's facts.

3. 3

   info!(seats, booking_id, "booking confirmed"), with % for Display and ? for Debug.

   Why: fields are what a log store can filter and count; values inside the message are only text.

4. 4

   error! for a failure someone must look at, warn! for something odd the service survived, info! for the main steps, debug! for detail.

   Why: production runs at info, so the level decides whether a line is seen at all.