What HTTP methods exist? Which of them are idempotent?

Main HTTP methods:

• GET — retrieve a resource.
• POST — create a resource or perform an operation.
• PUT — update or replace a resource.
• DELETE — delete a resource.
• PATCH — partially update a resource.
• HEAD — get resource headers.
• OPTIONS — get supported methods for a resource.

Idempotent methods are methods that can be executed multiple times without changing the result after the first call. These include:

• GET
• PUT
• DELETE
• HEAD
• OPTIONS

POST and PATCH are not idempotent, as repeated calls may lead to a change in state or creation of multiple resources.

An example of idempotency:

Calling DELETE /resource/1 multiple times will delete the resource after the first call, and subsequent calls will not change the system's state.