Idempotency and safe retries

Any request can be delivered twice. An idempotency key lets the second delivery return the first result instead of charging the card again.

What this lesson covers

• Make write endpoints safe to retry
• Accept and store idempotency keys
• Retry with backoff and jitter, not in lockstep

This lesson sits in API Design at Scale, part of System Design Foundations. It assumes what came before it and leads directly into the next lesson in the module.