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 requests.

This does not necessarily 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: for example, a POST request intends to send data to the server, or a DELETE request intends to delete a resource on the server.

All {{glossary("Safe/HTTP", "safe")}}  methods are idempotent, as well as {{HTTPMethod("PUT")}}  and {{HTTPMethod("DELETE")}} . The {{HTTPMethod("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 {{HTTPMethod("DELETE")}}  will likely return a {{HTTPStatus("200")}} , while successive ones will likely return a {{HTTPStatus("404")}} . Another implication of {{HTTPMethod("DELETE")}}  being idempotent 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 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 404 as it just got deleted
DELETE /idX/delete HTTP/1.1   -> Returns 404

See also

• Definition of idempotent in the HTTP specification.
• Description of common idempotent methods: {{HTTPMethod("GET")}} , {{HTTPMethod("HEAD")}} , {{HTTPMethod("PUT")}} , {{HTTPMethod("DELETE")}} , {{HTTPMethod("OPTIONS")}} , {{HTTPMethod("TRACE")}} 
• Description of common non-idempotent methods: {{HTTPMethod("POST")}} , {{HTTPMethod("PATCH")}} , {{HTTPMethod("CONNECT")}}