Idempotency Keys for Safe API Retries
Retries are essential in distributed systems. Networks fail, clients time out, load balancers reset connections, and responses sometimes disappear after a server has already committed a write.
The dangerous case is a retry of a non-idempotent operation. If a client sends POST /orders, times out, and sends the same request again, the server may create two orders even though the user intended one.
An idempotency key gives the client a stable identifier for one logical operation. The server remembers the result associated with that key and can return the same result when the request is retried.
