Definition

Idempotency means an operation can be applied multiple times without changing the result beyond the initial application. In distributed systems, it ensures retries don’t duplicate work.

Core concept

HTTP PUT is idempotent (setting value to X is same if done once or many times). Using unique request IDs or checks can achieve it.

Use cases

Exactly-once messaging, reliable REST APIs.

Trade-offs

Requires storage of seen requests or unique constraints.