What is idempotency? Which HTTP methods are idempotent?

Idempotence is a property of an operation where multiple executions with the same parameters produce the same result as a single execution.

In the context of HTTP methods, idempotent methods are those that can be called multiple times in succession without changing the server's state after the first call. These include:

• GET — retrieves a resource, does not change the state.
• PUT — updates a resource, repeated execution with the same data will not change the result.
• DELETE — deletes a resource, repeated deletion of the same resource will not change the state (the resource is already deleted).
• HEAD — similar to GET but without the response body, and is idempotent.
• OPTIONS — requests available methods, does not change the state.

The POST method is not idempotent, as repeated calls can create new resources or change the server's state.