Idempotent

Idempotency is a fundamental concept in computer science and API design that describes operations producing the same outcome regardless of how many times they are executed. In HTTP, GET, PUT, and DELETE methods are defined as idempotent, meaning repeated identical requests should have the same effect as a single request. POST is notably not idempotent, as each call may create a new resource.

Idempotency is critical in distributed systems where network failures, timeouts, and retries are common. Without idempotent operations, a client that retries a failed payment request might accidentally charge a customer twice. Implementing idempotency typically involves using unique idempotency keys that clients generate and include with each request. The server stores the result of the first successful request and returns that cached result for any subsequent requests with the same key.

Designing idempotent APIs requires careful consideration of state management and side effects. For database operations, using upsert patterns instead of separate insert and update logic naturally achieves idempotency. For operations with side effects like sending emails or processing payments, idempotency keys combined with deduplication logic ensure that retries do not cause unintended duplicate actions.