How would you implement an idempotent POST endpoint in Spring?

Places an idempotency key at the Spring boundary while persisting request identity and terminal outcome atomically with business work.

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TL;DR

The controller requires an idempotency key, validates its format, and passes it with a canonical request fingerprint to the application service. The service atomically claims the key in a durable store, performs the business transition, and records the terminal response or resource reference. A retry with the same key and same request returns the prior outcome. The same key with a different request is rejected as a conflict.

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HTTP boundary extraction, transaction ownership, concurrent retries, payload mismatch, replay, and ambiguous completion.

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Overview
A header and a map are not enough; idempotency is a persisted state machine around one logical operation.
The controller requires an idempotency key, validates its format, and passes it with a canonical request fingerprint to the application service. The service atomically claims the key in a durable store, performs the business transition, and records the terminal response or resource reference. A retry with the same key and same request returns the prior outcome. The same key with a different request is rejected as a conflict.
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A unique database constraint prevents two concurrent requests from both becoming the owner. The idempotency record and local business write share a transaction where possible; remote side effects require their own idempotency key and reconciliation. An in-memory cache is insufficient across restarts and multiple instances. In-progress, completed, failed, and expired states have explicit behavior rather than making a second request wait forever.

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Details

Keeping keys only in process memory.
Ignoring payload mismatch for a reused key.
Treating a downstream timeout as proof nothing happened.

How would you implement an idempotent POST endpoint in Spring?

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