Properties of HTTP Methods

Safe

Calling it does not change the resource.
Q: What if continuous calls accumulate logs and cause a failure?
A: Safety only considers the resource itself. It does not account for such side effects.

Idempotent

f(f(x)) = f(x)
Whether called once, twice, or 100 times, the result is the same.
Idempotent Methods
GET: Whether retrieved once or multiple times, the same result is returned.
PUT: Replaces the result. Therefore, even if the same request is made multiple times, the final result is the same.
DELETE: Deletes the result. Even if the same request is made multiple times, the deleted result is the same.
POST: Is not idempotent. Calling it twice can result in duplicate payments.
Usage
- Automatic recovery mechanism
- When the server fails to provide a normal response due to a TIMEOUT, etc., it serves as a basis for determining whether the client can retry the same request.
Q: What if another party modifies the resource during a retry?
- User1: GET > username : A age : 20
- User2: PUT > username : A age : 30
- User1: GET > username :A age : 30 -> Retrieves data changed by User2's action.
A: Idempotency does not account for resources being changed by external factors in the interim.

Cacheable

Can the response result resource be cached and used?
GET, HEAD, POST, PATCH are cacheable.
In practice, only GET and HEAD are typically used for caching.
For POST and PATCH, the request body content must also be considered as part of the cache key, which is not easy to implement.