Use Appropriate Request Methods
Every API MUST use appropriate HTTP request methods for every operation.
Every API designer, implementer and consumer MUST understand the semantic of the HTTP METHOD she is using.
At a minimum everyone MUST be familiar with the semantics of "Common" HTTP Request Methods: DELETE, GET, HEAD, PUT, POST and the PATCH HTTP Request Method. In addition, everyone MUST be aware which methods are Safe, Idempotent and Cacheable.
As per HTTP specification, the GET and HEAD methods should be used only for retrieval of resource representations – and they do not update/delete the resource on the server. Both methods are said to be considered “safe“. This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested – and they can update/delete the resource on the server and so should be used carefully.
The term idempotent is used more comprehensively to describe an operation that will produce the same results if executed once or multiple times. This is a beneficial property in many situations, as it means that a transaction can be repeated or retried as often as necessary without causing unintended effects. With non-idempotent operations, the algorithm may have to keep track of whether the operation was already performed or not. In HTTP specification, The methods GET, HEAD, PUT and DELETE are declared idempotent methods. Other methods OPTIONS and TRACE SHOULD NOT have side effects, so both are also inherently idempotent.
Request methods are considered "cacheable" if it is possible and useful to answer a current client request with a stored response from a prior request. GET and HEAD are defined to be cacheable.
GET /user/new Description: Creates new user
Using GET for unsafe non-idempotent operations is not acceptable.
POST /status Description: Updates the status of a user approval request (to “Approved” or “Rejected”)
Using the POST method for a status update is not acceptable (use PATCH).
PUT /user Description: Creates a new user
Using the PUT method for creating a new resource is not acceptable (use POST).
PUT: /user Description: Updates some user details
Using the PUT method for a partial update is not acceptable (use PATCH).