Idempotency key service
Design an idempotency-key service that makes mutating API calls (e.g. "charge this card", "create this order") safe under client retries, where the SAME key may arrive concurrently from a retry storm. Constraints: a payment must execute its side effect at most once even if 5 duplicate requests land within milliseconds, the client must get a consistent response across all duplicates, and a request that crashed mid-execution must be safely retryable. Describe the key store, how concurrent duplicates are coordinated, and what the second-arriving request sees.
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.