In the previous sections, we introduced the concepts of resources and representations. We learned that resources are mappings of the actual entity states that are exchanged between clients and servers. Further, we discussed that representations are negotiated between clients and servers through the communication protocol (HTTP) at runtime. In this section, you will learn about the generics of interaction semantics and self-descriptive messages followed for the client-server communication in a RESTful system.

Developing RESTful web services is similar to what we have been doing up to this point with our web applications. In a RESTful web service, resources are exchanged between the client and the server, which represent the business entities or data. HTTP specifies methods or actions for the resources. The most commonly used HTTP methods or actions are POST, GET, PUT, and DELETE. This clearly simplifies the REST API design and makes it more readable. On the other hand, in traditional application development, we can have countless actions with no naming or implementation standards. This may call for more development effort for both the client and the server and make the APIs less readable.

In a RESTful system, we can easily map our CRUD actions on the resources to the appropriate HTTP methods such as POST, GET, PUT, and DELETE. This is shown in the following table:

In fact, the preceding list of HTTP methods is incomplete. There are some more HTTP methods available, but they are less frequently used in the context of RESTful implementations. Of these less frequent methods, OPTIONS and HEAD are used more often than others. So, let's glance at these two method types:

• OPTIONS: This method is used by the client to determine the options or actions associated with the target resource, without causing any action on the resource or retrieval of the resource
• HEAD: This method can be used for retrieving information about the entity without having the entity itself in the response

In their simplest form, RESTful web services are networked applications that manipulate the state of resources. In this context, resource manipulation means resource creation, retrieval, updation, and deletion. However, RESTful web services are not limited to just these four basic data manipulation concepts. They can even be used for executing business logic on the server but remembering that every result must be a resource representation of the domain at hand.

A uniform interface brings all the aforementioned abstractions into focus. Consequently, putting together all these concepts, we can describe RESTful development with one short sentence--we use URIs to connect clients and servers in order to exchange resources in the form of representations.

Let's now look at the four HTTP request types in detail and see how each of them is used to exchange representations to modify the state of resources.