Idempotency keys in plain language

Retries happen on flaky networks, not because users are impatient. We teach idempotency keys as a story: first request stamps an intent, subsequent retries recognise the stamp and short-circuit safely. Labs use intentionally unreliable fake gateways so learners feel the pain. We also discuss clock skew, key rotation, and how long to store fingerprints — pragmatic windows, not infinite storage. The REST Design Patterns course ties this to API review culture: if your consumers cannot discover the rule in docs, the feature is not finished.