What is Idempotence?
Idempotence is a property of an operation in mathematics and computer science, where applying the same operation multiple times has the same effect as applying it exactly once.
In the context of HTTP, idempotence refers to the ability of an HTTP method (e.g., GET, PUT, DELETE) to be safely repeated multiple times without side effects.
To summarize, idempotence in HTTP means that a request can be repeated many times and have the same result as if it was executed once.
Idempotence is important in the design of RESTful APIs because it allows clients to retry failed requests without the risk of causing unintended consequences. For example, a GET request to a resource should always return the same result, regardless of how many times it is repeated, and a PUT request to update a resource should have the same effect, regardless of whether it is executed once or multiple times.
HTTP method and Idempotence
GET : idempotent
The primary purpose of the GET method is to retrieve data or resources from a server. It does not involve any modifications or side effects on the server's state. Repeating the same GET request multiple times will always retrieve the same information from the server without altering the state of the server or the requested resource.
Q.
What if, while a GET request is being processed, a POST request completes its task and changes the server state? In this scenario, the subsequent GET request may return a different response compared to the previous one. Nevertheless, can the GET request still be considered idempotent?
A.
YES.
According to HTTP 1.1 RFC Document (Section 4.2.2: Idempotent Methods), similar to safe methods, the property of being idempotent applies specifically to the requested action by the user. This means that the server is permitted to handle each request separately, including actions such as logging each request individually, maintaining a revision control history, or implementing other non-idempotent side effects for each idempotent request.
Therefore, even if a GET request returns a different result on different executions, it is still considered idempotent. The reason is that executing a GET request multiple times should not have any side effects on the server state; its purpose is solely to retrieve information from the server.
If a GET request yields a different result due to changes in the server state caused by other operations (e.g., a POST request), it is still regarded as idempotent. However, it is the responsibility of the client to handle such changes in the server state and update its own state accordingly.
PUT : idempotent
The PUT method is used to update or create a resource at a specific URL. If the same PUT request is executed multiple times, it will result in the same final state of the resource because the PUT method replaces the entire existing resource on the server with the contents specified in the request. Each execution of the PUT request replaces the current state of the resource with the new representation provided in the request payload, ensuring that the final state of the resource is consistent regardless of how many times the request is repeated.
DELETE : idempotent
The DELETE method is used to remove a specific resource from the server. If a DELETE request is repeated multiple times, the resource will be deleted only once. Subsequent DELETE requests on the same resource will have no effect since the resource has already been removed.
HEAD : idempotent
The HEAD method is similar to a GET request, but it only retrieves the header information of a resource without returning the response body. Since the HEAD request does not modify the server state, repeating it multiple times will always return the same header information for the requested resource.
OPTIONS : idempotent
The OPTIONS method is used to retrieve the communication options available for a resource. It is also considered idempotent because it only retrieves information about the resource and does not make any changes to it.
POST : non-idempotent
The POST method is used to submit data to be processed by the server, typically resulting in the creation of a new resource. Each time a POST request is made, it typically leads to a new resource being created on the server. Repeating the same POST request multiple times will result in the creation of multiple resources with potentially different identifiers or states. Therefore, the effect of multiple identical POST requests is not the same as a single request, making it non-idempotent.
PATCH : non-idempotent
The PATCH method is used to partially update an existing resource on the server. It allows modifying specific fields or properties of a resource without affecting the entire resource. Unlike the PUT method, which completely replaces the resource, PATCH only updates specific portions. Repeating a PATCH request multiple times can result in cumulative changes to the resource. The final state of the resource depends on the sequence and content of the patch operations, making it non-idempotent.
