Sommerville-Software-Engineering-10ed

530 Chapter 18 ■ Service-oriented software engineering
CREATE
POST
URL
DELETE
Resource R
READ
DELETE
Web-accessible
resource R
GET
UPDATE
PUT
Figure 18.6 Resources
and actions
(a) General resource actions
(b) Web resources
style, where REST stands for Representational State Transfer (Fielding 2000). REST
is an architectural style based on transferring representations of resources from a
server to a client. It is the style that underlies the web as a whole and has been used as
a much simpler method than SOAP/WSDL for implementing web service interfaces.
The fundamental element in a RESTful architecture is a resource. Essentially, a
resource is simply a data element such as a catalog and a medical record, or a document,
such as this book chapter. In general, resources may have multiple representations;
that is, they can exist in different formats. For example, this book chapter has
three representations. These are a MS Word representation, which is used for editing,
a PDF representation, which is used for web display, and a InDesign representation,
which is used for publishing. The underlying logical resource made up of text and
images is the same in all of these representations.
In a RESTful architecture, everything is represented as a resource. Resources have
a unique identifier, which is their URL. Resources are a bit like objects, with four fundamental
polymorphic operations associated with them, as shown in Figure 18.6(a):
1. Create—bring the resource into existence.
2. Read—return a representation of the resource.
3. Update—change the value of the resource.
4. Delete—make the resource inaccessible.
The Web is an example of a system that has a RESTful architecture. Web pages
are resources, and the unique identifier of a web page is its URL.
The web protocols http and https are based on four actions, namely, POST, GET,
PUT, and DELETE. These map onto the basic resource operations, as I have shown
in Figure 18.6(b):
1. POST is used to create a resource. It has associated data that defines the resource.
2. GET is used to read the value of a resource and return that to the requestor in the
specified representation, such as XHTML, that can be rendered in a web browser.