In the context of web services, particularly RESTful APIs, idempotent methods are those that can be called multiple times without changing the result beyond the initial application. This means that making the same request multiple times will have the same effect as making it just once. The primary idempotent HTTP methods are:
GET: This method is used to retrieve data from the server. It is idempotent because calling it multiple times will not change the state of the resource on the server; it simply returns the requested data each time[1][3][4][5][6][10][11].
PUT: This method is used to update or create a resource. It is idempotent because sending the same PUT request multiple times will result in the resource being in the same state as after the first request. If the resource already exists, it will be updated; if it does not exist, it will be created[1][3][4][5][6][10][11].
DELETE: This method is used to remove a resource. It is idempotent because once the resource is deleted, subsequent DELETE requests will not change the state further. The first DELETE request will remove the resource, and subsequent requests will simply confirm that the resource is no longer there[1][3][4][5][6][10][11].
HEAD: This method is similar to GET but only retrieves the headers of a resource, not the body. It is idempotent because it does not change the state of the resource on the server[1][3][4][5][6][10][11].
OPTIONS: This method is used to describe the communication options for the target resource. It is idempotent because it does not change the state of the resource on the server[1][3][4][5][6][10][11].
TRACE: This method is used for diagnostic purposes and echoes back the received request. It is considered idempotent because it does not change the state of the resource on the server[1][3][4][5][6][10][11].
