Tell about HTTP methods and their idempotency.

HTTP methods define the action that the client wants to perform on a resource. Main methods:

• GET: Retrieve a resource. Idempotent (repeated requests do not change server state).
• POST: Send data for processing (e.g., create a new resource, submit form data). Usually not idempotent.
• PUT: Create or update a resource at the specified URI. Idempotent (repeated request with same data results in same resource state).
• DELETE: Delete the resource at the specified URI. Idempotent (subsequent requests do not change state after first success).
• PATCH: Partially modify a resource. Usually not idempotent.
• HEAD: Get response headers without body. Idempotent.
• OPTIONS: Describe communication options for the resource. Idempotent.

Idempotency means that executing the same request multiple times results in the same system state as executing it once. Responses may differ (e.g., first DELETE returns 200 OK, subsequent return 404 Not Found).

Examples of non-idempotent operations:

• Repeatedly adding an item to cart via POST.
• Incrementing a counter via POST or PATCH.

Examples of idempotent operations:

• Repeatedly fetching data with GET.
• Creating or updating a user with a specific ID via PUT.
• Deleting a user with a specific ID via DELETE.

Idempotency is important for safe retries, e.g., in case of network failures. Servers can safely repeat idempotent requests without causing unwanted changes.