/ The Arena

Landmark Capstone: Distributed Idempotency Engine

In distributed systems, network partitions and client retries can result in catastrophic duplicate transactions. Your objective is to build a production-grade Idempotency Middleware backed by Redis.

Requirements:

1. Cache Hit: If a request with idempotencyKey has already been executed, immediately return the cached status and payload with isIdempotentHit: true.
2. Concurrent Lock Acquisition: Acquire an atomic distributed lock (SET NX EX). If another worker is actively executing the same key, return HTTP 409 Conflict and { error: 'CONCURRENT_REQUEST_IN_PROGRESS' }.
3. Payload Fingerprint Validation: If a subsequent request reuses an existing idempotencyKey with a mismatched HTTP method, path, or body payload, return HTTP 422 Unprocessable Entity and { error: 'IDEMPOTENCY_KEY_PAYLOAD_MISMATCH' }.
4. Atomic Commitment & Safe Unlock: Commit the response to Redis with a 24-hour TTL and release the lock to guarantee safe concurrent execution.