Concept-Check: The Service

Kind: Quiz. One pass over the whole arc before you move on.

The Service arc gave the coordinator a front door: an axum Router mapping paths to handlers, extractors (State, Path, Json) that hand each handler typed pieces of the request, a single ApiError IntoResponse impl where the ControlError taxonomy becomes an HTTP status code, and a test style that spawns the real server on 127.0.0.1:0 and drives it with reqwest. This check mixes a compiler-verified tracing question with judgment questions across all of it.

If a question stings, the fix is upstream: re-read axum — Handlers, State, Extractors, IntoResponse for the extractor-order and IntoResponse questions, and the Build: The Coordinator API spec for the validation and status-mapping details. The theme to carry forward: the type taxonomy maps to HTTP status in one place, and the store is the source of truth.

Next: Part VI, the Scheduler arc — bounded concurrency, retrying only the retryable, the LocalWorker, and graceful shutdown, where the coordinator stops being a request/store layer and becomes a running program that actually executes the queued runs.