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Design Idempotency Keys for APIs: Safe Retries Without Duplicate Charges or Orders

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Self-Hosting & Infra advanced 8 min read $0.01 Updated 2026-10-19

Designing idempotent write endpoints: key generation on clients, a unique-constrained key table, request fingerprinting, in-progress locking, replaying stored responses, 409 and 422 behavior, TTLs, natural idempotency, and testing with parallel requests.

Retries create duplicate payments, orders or emails. Add an Idempotency-Key header, store the first result atomically, handle concurrent duplicates and mismatched payloads, and expire keys sensibly.

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#idempotency#api-design#retries#payments#webhooks

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