REST HTTP methods: what are they, what is each used for, which are idempotent, which are not?

Main HTTP methods in REST and their purpose:

• GET — retrieve a resource. Idempotent and safe method, should not change the server state.
• POST — create a resource or perform an operation. Not idempotent, repeated calls may create duplicates.
• PUT — update or replace a resource. Idempotent: repeated calls with the same data will not change the result.
• PATCH — partial update of a resource. Usually idempotent, but depends on implementation.
• DELETE — delete a resource. Idempotent: repeated deletion of the same resource will not change the state.
• HEAD — retrieve only the headers of a resource, without the body. Idempotent and safe.
• OPTIONS — get the supported methods for a resource from the server.

Idempotent methods: GET, PUT, DELETE, HEAD, OPTIONS, PATCH (usually).

Non-idempotent method: POST.

Example:

• GET /users/123 — get user with ID 123
• POST /users — create a new user
• PUT /users/123 — replace data of user 123
• PATCH /users/123 — update part of user 123's data
• DELETE /users/123 — delete user 123

Idempotency is important for reliability: if a request is repeated (for example, due to network failures), the server state will not change unpredictably.