Logs describe events, metrics summarize behavior, and traces connect work across services.

How to apply it

Use structured logs with timestamps, severity, request IDs, trace IDs, and safe context. Monitor request rate, errors, latency percentiles, saturation, queue depth, worker failures, and customer-facing success rates. Propagate trace context through jobs.

Worked example

A distributed trace reveals that a slow API request spends most of its time in a database query.

Common mistakes to avoid

Logging secrets or measuring CPU while ignoring customer outcomes.

Practice task

Implement this concept in the capstone. Add one positive test, one negative or failure-path test, and a short note explaining the trade-off you chose.