> For the complete documentation index, see [llms.txt](https://docs.zumrails.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.zumrails.com/api-reference/idempotency.md).

# Idempotency

In Zūm APIs, idempotence means that requests that have been previously processed successfully are not reprocessed again. The response for the completed processing is reported instead.

All `POST` requests accept idempotence keys. Sending idempotence keys in `GET` and `DELETE` requests will have no effect, as these type of requests are idempotent by definition.

To submit a request with idempotent processing, add the key `idempotency-key` in the header. The value for this key should be a unique identifier for the message with a maximum of 36 characters (we recommend a UUID). If you don't receive a response (for example, in case of a timeout), you can safely retry the request with the same header. If we have already processed the request, the response to the first attempt will be returned without duplicating the request in our database.

Idempotence keys are persisted in Zūm Rails side for 1 hour, which means if you send the same key after 1 hour, the request will be handled as a new request. If the same idempotence key is used on two different transactions within 1 hour of each other, the second transaction will not be created.


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