Telemetry Essentials

RULES — Follow these with no exceptions

1. Use structured logging (Logger.info("action", key: value)) — never string interpolation in log messages; structured logs are searchable and parseable
2. Attach telemetry handlers in Application.start/2 — :telemetry.attach/4 with an existing handler ID returns {:error, :already_exists}, so it never double-attaches; attach at boot anyway, since handlers detach permanently if they raise and boot-time attachment gives them a stable lifecycle
3. Use Ecto.Repo telemetry events for query monitoring — don't wrap every query manually; Ecto already emits events
4. Use Phoenix.LiveDashboard in dev/staging — it's free observability with zero code
5. Tag telemetry events with metadata (user_id, request_id) — without correlation IDs, distributed traces are useless
6. Never log at :debug level in production — it includes query parameters and PII

Structured Logging

Structured logs can be filtered, searched, and aggregated. String-interpolated logs cannot.