Idempotency Keys: The API Pattern That Prevents Duplicate Charges (and Worse)

The article explains the idempotency-key pattern for safely retrying non-idempotent API requests.

A client reuses one unique key on retries, while the server stores the first response and returns it for duplicates.

The implementation uses a lock and TTL to prevent concurrent reprocessing and keep retries consistent.

This is especially important for payments, where duplicate charges and other repeat side effects must be avoided.