A Spring Boot starter library that adds idempotent REST endpoint handling via a single `@Idempotent` annotation. Duplicate requests carrying the same `Idempotency-Key` header get the original response replayed rather than re-executed. Supports Redis and JDBC/Postgres as backing stores with atomic claim semantics (SETNX or INSERT ON CONFLICT). Covers the full failure state machine: 5xx errors release the key for retry, 4xx responses are cached and replayed, and body fingerprinting catches mismatched payloads. Concurrent duplicate requests are handled via polling or fail-fast modes. The post is transparent about the at-least-once vs. exactly-once distinction, explaining the AOP boundary limitation and how to achieve exactly-once with the JDBC store. Benchmarks show ~5–6ms p50 and ~40ms p99 added latency from two Redis/Postgres round trips. Extensible via custom `IdempotencyStore`, `ScopeResolver`, and `IdempotencyMetrics` interfaces.
