What was used as the idempotency key?

An example of an idempotency key is a unique request or transaction identifier, such as a UUID, generated on the client or server side during the first call of an operation.

This allows the system to recognize repeated requests with the same key and avoid executing the action again, instead returning the result of the previous execution.

For example: when purchasing an in-game item, the client sends a request with a unique transactionId. If the request is repeated due to network issues, the server checks this key and does not charge twice.