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Idempotency: Preventing Duplicate Operations
How to design and implement end-to-end API idempotency using Idempotency Keys and distributed Redis locks — preventing double billing, duplicate order creation, and webhook replay race conditions.
Over an unreliable network, timeouts are ambiguous. A client timeout does not mean the server failed; it may mean the request succeeded and only the response was lost. Without idempotency, automatic retries cause duplicate state mutations.
Require an `Idempotency-Key` header on all state-mutating POST endpoints (payments, orders, transfers). Store idempotency records in Redis with a 24-hour TTL and hash the request payload to detect tampering.
- Understand why at-least-once network delivery makes duplicate API requests inevitable
- Master the 5-step Idempotency Key state machine (Locking, Executing, Storing, Playback)
- Implement atomic distributed locking in Redis using SET NX PX and Lua scripts
- Handle edge cases: concurrent in-flight duplicates and payload mismatch tampering
1. The Ambiguity of Network Timeouts
Free Chapter PreviewConsider the life of a financial API request:
- 1Client sends
POST /api/v1/charges($100). - 2Server deducts $100 from database and creates invoice.
- 3Server attempts to write the HTTP response back to the client.
- 4Cellular connection drops. The client socket reads a
TIMEOUTerror.
From the client's perspective: Did the charge succeed or fail? If the client automatically retries, a naive non-idempotent server charges the user another $100. Duplicate payments are the single most expensive customer support disaster in fintech.
2. The Idempotency Key Specification
In this section, we examine the production failure mechanics, internal socket buffers, and kernel-level trace diagnostics...
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