Once you have identified all the resources, as the next step, you may want to map the operations defined on the resources to the appropriate HTTP methods.

The most commonly used HTTP methods (verbs) in RESTful web APIs are POST, GET, PUT, and DELETE. Note that there is no one-to-one mapping between the CRUD operations defined on the resources and the HTTP methods. Understanding of idempotent and safe operation concepts will help with using the correct HTTP method.

Check out the following table:

Here are some tips for identifying the most appropriate HTTP method for the operations that you want to perform on the resources:

• GET: You can use this method for reading a representation of a resource from the server. According to the HTTP specification, GET is a safe operation, which means that it is only intended for retrieving data, not for making any state changes. As this is an idempotent operation, multiple identical GET requests will behave in the same manner.

A GET method can return the 200 OK HTTP response code on the successful retrieval of resources. If there is any error, it can return an appropriate status code such as 404 NOT FOUND or 400 BAD REQUEST.

• DELETE: You can use this method for deleting resources. On successful deletion, DELETE can return the 200 OK status code. According to the HTTP specification, DELETE is an idempotent operation. Note that when you call DELETE on the same resource for the second time, the server may return the 404 NOT FOUND status code since it was already deleted, which is different from the response for the first request. The change in response for the second call is perfectly valid here. However, multiple DELETE calls on the same resource produce the same result (state) on the server.
• PUT: According to the HTTP specification, this method is idempotent. When a client invokes the PUT method on a resource, the resource available at the given URL is completely replaced with the resource representation sent by the client. When a client uses the PUT request on a resource, it has to send all the available properties of the resource to the server, not just the partial data that was modified within the request.

• POST: This method is not idempotent. This method enables you to use the POST method to create or update resources when you do not know all the available attributes of a resource. For example, consider a scenario where the identifier field for an entity resource is generated at the server when the entity is persisted in the data store. You can use the POST method for creating such resources as the client does not have an identifier attribute while issuing the request. Here is a simplified example that illustrates this scenario. In this example, the employeeID attribute is generated on the server:

POST hrapp/api/employees HTTP/1.1 
Host: packtpub.com 
{employee entity resource in JSON} 

• On the successful creation of a resource, it is recommended to return the status of 201 Created and the location of the newly created resource. This allows the client to access the newly created resource later (with server-generated attributes). The sample response for the preceding example will look as follows:

201 Created 
Location: hrapp/api/employees/1001