Chapter 3. Web Services: A Realization of SOA

People often think of Web services and Service-Oriented Architecture (SOA) in combination, but they are distinct in an important way. As discussed in Chapter 1, "Service-Oriented Architectures," SOA represents an abstract architectural concept. It's an approach to building software systems that is based on loosely coupled components (services) that have been described in a uniform way and that can be discovered and composed. Web services represents one important approach to realizing an SOA.

The World Wide Web Consortium (W3C), which has managed the evolution of the SOAP and WSDL specifications, defines Web services as follows:

A software system designed to support interoperable machine-to-machine interaction over a network. It has an interface described in a machine-processable format (specifically WSDL). Other systems interact with the Web service in a manner prescribed by its description using SOAP messages, typically conveyed using HTTP with XML serialization in conjunction with other Web-related standards.

Although Web services technology is not the only approach to realizing an SOA, it is one that the IT industry as a whole has enthusiastically embraced. With Web services, the industry is addressing yet again the fundamental challenge that distributed computing has provided for some considerable time: to provide a uniform way of describing components or services within a network, locating them, and accessing them. The difference between the Web services approach and traditional approaches (for example, distributed object technologies such as the Object Management Group Common Object Request Broker Architecture (OMG CORBA), or Microsoft Distributed Component Object Model (DCOM)) lies in the loose coupling aspects of the architecture. Instead of building applications that result in tightly integrated collections of objects or components, which are well known and understood at development time, the whole approach is much more dynamic and adaptable to change. Another key difference is that through Web services, the IT industry is tackling the problems using technology and specifications that are being developed in an open way, utilizing industry partnerships and broad consortia such as W3C and the Organization for the Advancement of Structured Information Standards (OASIS), and based on standards and technology that are the foundation of the Internet.

This open, standards-based approach in which every Web services specification is eventually standardized by an industry-wide organization (such as W3C or OASIS) introduces the possibility that the specifications described in this book might undergo significant changes before becoming formal standards. This is a natural consequence of the standardization process in which both technology vendors and consumers provide input and push their requirements into the final standard. However, the basic concepts and the design supporting each of the specifications are unlikely to change in fundamental ways, even if the syntax is modified or the supported set of use cases is significantly expanded. At the time of publication, several of the specifications covered in this book have already been submitted to standards, and significant changes may ensue in some of them (for example, in the case of WS-Addressing, now being discussed at W3C). Readers interested in the details of the specifications should be aware of this fact and carefully follow the results of the standardization process. Please refer to the Web site, www.phptr.com, "Updates and Corrections," where you will find the latest updates to the specifications covered in this book.