Service-Oriented Architecture - Guide to Cloud Computing for Business and Technology Managers - page 162

7.4.1.3 Resources
The term resource is used here to specifically refer to the abstraction used by
the Web and related initiative such as the Semantic Web. Such a resource is
different from a distributed object in a number of ways:
Resource state is not hidden from a client as it is in object systems. Instead,
standard representations of state are exposed. In object systems, the public
interface of an object gives access to hidden state:
• Unlike distributed objects, resources do not have operations asso-
ciated with them. Instead, manipulation and retrieval of resource
representations rely on the transfer protocol used to dereference the
uniform resource identifier (URI).
• As a consequence, a resource can be viewed as an entity that has
state, but not the logic to manipulate that state, that is, no behavior.
Because resources have no behavior, they do not define how their state can
be manipulated. While this could be viewed as limiting and potentially
leading to ad hoc, underspecified interactions, in the case of the Web, the
opposite is actually true. While an object-oriented system defines propri-
etary behavioral interfaces for every object, leading to a proliferation of
means of manipulating objects, the Web uses a single, shared interface:
HTTP. The few methods defined by HTTP allow arbitrary resources to
be exchanged and manipulated, making interactions between entities far
simpler and hence scalable. Imagine, for example, that every Web server
defined its own interface to accessing the resources in its charge. This
would require a browser to digest a new service interface and generate
client-side code every time you clicked on a link, a process that would
severely influence the scalability of the system as a whole.
7.4.2 SOA and Web Services
Web Services are new standards for creating and delivering cooperative
applications over the Internet. The basic Web Service architecture consists
of specifications (SOAP, WSDL, and UDDI) that support the interaction of
a Web Service requester with a Web Service provider and the potential dis-
covery of the Web Service description. The provider is typically publishing a
WSDL description of its Web Service, and the requester accesses the descrip-
tion using a UDDI or other type of registry and requests the execution of the
provider's service by sending a SOAP message to it. The basic Web Service
standards are good for some SOA-based applications but not adequate for
many others.
SOAP, originally defined as Simple Object Access Protocol is a protocol
specification for exchanging structured information in the implementa-
tion of Web Services in computer networks. It relies on XML as its message