Idempotent

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An HTTP method is idempotent if the intended effect on the server of making a single request is the same as the effect of making several identical requests1. Calling it once is no different from calling it several times successively i.e. there are no side effects 2.

This does not mean that the request does not have any unique side effects. For example, the server may log every request with the time it was received. Idempotency only applies to effects intended by the client, e.g. a POST request intends to send data to the server, or a DELETE request intends to delete a resource on the server.

All safe methods are idempotent, as well as PUT and DELETE . The POST method is not idempotent.

To be idempotent, only the state of the server is considered. The response returned by each request may differ: for example, the first call of a DELETE will likely return a 200, while successive ones will likely return a 404. Another implication of DELETE being idempotend is that developers should not implement RESTful APIs with a delete last entry functionality using the DELETE method .

Note that the idempotence of a method is not guaranteed by the server and some applications may incorrectly break the idempotence constraint.

GET /pageX HTTP/1.1 is idempotent, because, because it is a safe (read-only ) method . Successive calls may return different data to the client , if the data on the server was updated in the meantime.

POST /add_row HTTP/1.1 is not idempotent; if it is called several times, it adds several rows:

POST /add_row HTTP/1.1
POST /add_row HTTP/1.1 -> Adds a 2nd row
POST /add_row HTTP/1.1 -> Adds a 3rd row

DELETE /idX/delete HTTP/1.1 is idempotent, even if the returned status code may change between requests:

DELETE /idX/delete HTTP/1.1 -> Returns 200 if idX exists
DELETE /idX/delete HTTP/1.1 -> Returns 4004 as it just got deleted
DELETE /idX/delete HTTP/1.1 -> Returns 404

Anki

References

  1. MDN. “Idempotent”. Available at: https://developer.mozilla.org/en-US/docs/Glossary/Idempotent . (Accessed: [2025-05-06 Tue 21:11]). ↩︎

  2. MDN. “POST”. Available at: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Methods/POST . (Accessed: [2025-05-06 Tue 21:11]). ↩︎

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