What is the purpose of an idempotency key in a payment API?

• A. Allow servers to detect and deduplicate retries so a request is only processed once.
• B. Identify the user across sessions.
• C. Provide audit logging for retries.
• D. Increase the rate of retries.

Answer: (A)

An idempotency key lets the server recognize repeated attempts of the same operation and ensure it only processes it once. In a payment API, a client might retry a payment after a timeout or network hiccup. Without this safeguard, sending the same request again could charge the customer twice. The client generates a unique idempotency key for that operation and includes it with the request. The server stores the outcome of the first request tied to that key; if a subsequent request arrives with the same key, the server returns the stored result instead of processing the operation again. This preserves a single, consistent outcome and prevents duplicate charges, even in the face of retries.

This idea is different from identifying a user across sessions, which is about authentication and user state. Audit logging can accompany idempotency to trace what happened, but the main purpose is safe retry handling. It also doesn’t aim to increase retries; it focuses on making retries harmless and predictable.