what do you mean by Idempotent in REST APIs?

Problem statement: what do you mean by idempotent and/or safe methods in Rest APIs?
* idempotent = unchanged

In context of REST APIs, when making multiple identical requests has the same effect as making a single request – then that REST API is called idempotent.

When you design REST APIs, you must realize that API consumers can make mistakes. They can write client code in such a way that there can be duplicate requests as well. These duplicate requests may be unintentional as well as intentional some time (e.g. due to timeout or network issues). You have to design fault-tolerant APIs in such a way that duplicate requests do not leave the system unstable.

An idempotent HTTP method is a HTTP method that can be called many times without different outcomes. It would not matter if the method is called only once, or ten times over. The result should be the same. It essentially means that the result of a successful performed request is independent of the number of times it is executed. For example, in arithmetic, adding zero to a number is idempotent operation.

If you follow REST principles in designing API, you will have automatically have idempotent REST APIsfor GET, PUT, DELETE, HEAD, OPTIONS and TRACE http methods. Only POST APIs will not be idempotent.

1. POST is NOT idempotent.
2. GET, PUT, DELETE, HEAD, OPTIONS and TRACE are idempotent.

Let’s analyze how above HTTP methods end up being idempotent – any why POST is not.

HTTP POST

Generally – not necessarily – POST APIs are used to create a new resource on server. So when you invoke the same POST request N times, you will have N new resources on server. So, POST is not idempotent.

HTTP GET, HEAD, OPTIONS and TRACE

GET, HEAD, OPTIONS and TRACE methods NEVER change the resource state on server. They are purely for retrieving the resource representation or meta data at that point of time. So invoking multiple requests will not have any write operation on server, so GET, HEAD, OPTIONS and TRACE are idempotent.

HTTP PUT

Generally – not necessarily – PUT APIs are used to update the resource state. If you invoke a PUT API N times, very first request will update the resource; then rest N-1 requests will just overwrite the same resource state again and again – effectively not changing anything. Hence, PUT is idempotent.

HTTP DELETE

When you invoke N similar DELETE requests, first request will delete the resource and response will be 200 (OK) or 204 (No Content). Other N-1 requests will return 404 (Not Found). Clearly, the response is different from first request, but there is no change of state for any resource on server side because original resource is already deleted. So, DELETE is idempotent.

Please keep in mind if some systems may have DELETE APIs like this:

DELETE /item/last

In above case, calling operation N times will delete N resources – hence DELETE is not idempotent in this case. In this case, a good suggestion might be to change above API to POST – because POST is not idempotent.

POST /item/last