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Chapter 3. Simple Object Access Protocol (SOAP)


There is a lot more to Web services than Simple Object Access Protocol (SOAP). Chapter 1, "Web Services Overview," introduced the Web services interoperability stack that went several levels higher than SOAP. SOAP is synonymous with Web services, however, because since its introduction in late 1999, it has become the de facto standard for Web services messaging and invocation. With competitive and market pressures driving the Web services industry in a hard race to provide meaningful solutions to cross-enterprise integration problems, SOAP is the go-to-market technology of choice.

What is SOAP all about, you ask? Will it save you from failure (and keep you clean) while you toil 80-hour work weeks on a business-to-business (B2B) integration project from hell? Will it support your extensibility needs as requirements change, and provide you with interoperability across multi-vendor offerings? Will it be the keyword on your resume that will guarantee you a big raise as you switch jobs? In short, is it the new new thing? Well, maybe.

SOAP is so simple and so flexible that it can be used in many different ways to fit the needs of different Web service scenarios. This is both a blessing and a curse. It is a blessing because chances are that SOAP can fit your needs. It is a curse because you probably won't know how to make it do that. This is where this chapter comes in. When you are through with it, you will know not only how to use SOAP straight out of the box, but also how to extend SOAP in multiple ways to support your diverse and changing needs. You will also have applied design best practices to build several meaningful e-commerce Web services for our favorite company, SkatesTown. Last but not least, you will be ready to handle the rest of the book and climb still higher toward the top of the Web services interoperability stack. To this end, the chapter will discuss the following topics:

  • The evolution of XML protocols and the history and motivation behind SOAP's creation

  • The SOAP envelope framework, complete with discussions of versioning, header-based vertical extensibility, intermediary-based horizontal extensibility, error handling, and bindings to multiple transport protocols

  • The various mechanisms for packaging information in SOAP messages, including SOAP's own data-encoding rules and a number of heuristics for putting just about any kind of data in SOAP messages

  • The use of SOAP within multiple distributed system architectures such as RPC-and messaging-based systems in all their flavors

  • Building and consuming Web services using the Java-based Apache Axis Web services engine

One final note before we begin. The SOAP 1.1 specification is slightly over 40 pages long. This chapter is noticeably longer, because the purpose of this book is to be something more than an annotated spec or a tutorial for building Web services. We've tried hard to create a thorough treatment of Web services for people who want answers to questions that begin not only with "what" and "how" but also with "why." To become an expert at Web services, you need to be comfortable dealing with the latter type of questions. We are here to help.

So, why SOAP? As this chapter will show, SOAP is simple, flexible, and highly extensible. Because it is XML based, SOAP is programming language, platform, and hardware neutral. What better choice for the XML protocol that is the foundation of Web services? To prove this point, let's start the chapter by looking at some of the earlier work that inspired SOAP.

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