Idempotency Keys: The Simple Trick That Prevents Duplicate Payments and Ghost Orders
You've seen it happen. A user clicks "Place Order," the network hiccups, the request times out, and now they've been charged twice. Or the charge went through but the order never appeared. Neither outcome is acceptable, and both are preventable — with idempotency keys.
What Is an Idempotency Key?
An idempotency key is a unique identifier you attach to a request so that the server can safely process it exactly once, no matter how many times the client retries. If the server receives the same key again, it returns the original response instead of executing the operation a second time.
The concept is borrowed from mathematics: an operation is idempotent if applying it multiple times produces the same result as applying it once. DELETE /users/42 is naturally idempotent — deleting an already-deleted user changes nothing. POST /orders is not — sending it twice creates two orders.
