Contrary to popular belief, REST is not a protocol, but an architectural principle for managing state information. It's mainly used in web applications. REST was introduced by Roy Fielding to overcome implementation difficulties in SOAP. Roy's doctoral dissertation made for an easy way to retrieve data, regardless of the platform used. You will see all the components of RESTful web services in the following sections.
In REST principles, all resources are identified by the Uniform Resource Identifier (URI).
HTTP REST resources are represented in some media types, such as XML, JSON, and RDF. Also, RESTful resources are self-descriptive, which means enough information is given to describe how to process the request.
In another REST principle, the clients interact with servers through hypermedia, which is dynamically provided by the servers. Other than endpoints, clients don't need to know how to interact with RESTful services. This principle is referred to as Hypermedia as the Engine of Application State (HATEOAS).
By separating REST entities such as the client and server, we can reduce the complexity of REST principles, which will show clear boundaries between server and client. This decoupling will help developers concentrate on the client and server independently. Also, it will help to manage different roles for the client and server.
In REST principles, the server will not keep any state about the client session on the server side; hence, it's stateless. If two calls are made to the server from a single client, the server will not identify whether both the calls are from the same client or not. As far as the server knows, every request is independent and new. Based on the URL, HTTP headers, and request body, including the parameters, the operation might be changed on the server side.
With RESTful web services, a client can cache any response coming from the server. The server can mention how, and for how long, it can cache the responses. With the caching option, a client can use the responses instead of contacting the server again. Also, caching will improve scalability and performance by avoiding client-server interactions all the time.
Note
This principle has significant advantages for scalability. Caching techniques will be discussed in Chapter 8, Performance.
Since REST typically leverages HTTP, it inherits all the caching properties that HTTP offers.
By providing the layered system, a server can hide its identity. By doing this, clients won't know which server they are dealing with. This policy gives more security control by providing intermediate servers and supports the load-balancing feature, too. Also, intermediate servers can improve scalability and performance through load-balancing and shared caches.
Code on demand (COD) is considered an optional principle. Servers can extend the functionality of clients by transferring executable code. For example, JavaScript can be provided to web-based clients to customize the functionality. As code on demand reduces the visibility of the client side, this constraint is optional. Also not all APIs need this feature.
In web applications, REST is typically used over HTTP. REST doesn't need to be tied to any specific protocol. In HTTP REST, we mainly use the GET, POST, PUT, and DELETE methods to change the state of the resources we access. Other HTTP methods, such as OPTIONS, HEAD, CONNECT, and TRACE, can be used for more advanced operations, for example, for caching and debugging purposes. Most servers have disabled advanced methods for security and simplicity reasons; however, you can enable them by adjusting the server configuration files. As JSON is used as a primary media type for major applications, we also use only the JSON media type for our web service calls.