Designing idempotent APIs: why it matters and how to do it right

Idempotency is one of those concepts that seems academic until a duplicate payment goes through or a user gets double-charged. An idempotent API guarantees that making the same request multiple times produces the same result as making it once. This is critical for any operation that creates, updates, or transfers resources.

The simplest implementation is an idempotency key. The client generates a unique key for each request and sends it as a header. The server checks if it has seen this key before. If it has, it returns the previous response without executing the operation again. If not, it processes the request and stores the result keyed by the idempotency key.

Store idempotency keys in a database with a unique constraint. Set a TTL on them you don't need to remember idempotency keys forever. A 24-hour window covers most retry scenarios. After the TTL expires, the same key can be reused safely.

Design your API with idempotent operations by default. GET, PUT, and DELETE are naturally idempotent. POST requires explicit idempotency support. PATCH is tricky because partial updates can be non-idempotent prefer PUT for updates or use conditional PATCH with version numbers.

Handle retries explicitly on the server side. If the server receives a request with an idempotency key but hasn't completed processing yet, return a 409 Conflict or 429 Too Many Requests. The client should retry with exponential backoff.

Test idempotency thoroughly. Simulate network retries, duplicate requests, and concurrent requests with the same idempotency key. These tests catch subtle race conditions that can lead to data corruption.

Idempotency is a contract between client and server. Document it clearly in your API spec. Tell clients which endpoints are idempotent, how to generate idempotency keys, and what response codes to expect for duplicate requests.

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Rizwan Saleem | https://rizwansaleem.co