SOAP

Having understood the SOAP model and seen how this model is supported by SOAP servers, we can now begin to discuss the details of SOAP itself. SOAP is the oldest, most mature, and the single most important protocol in the Web services world. The SOAP specification defines this protocol as "[an] XML-based protocol that consists of three parts: an envelope that defines a framework for describing what is in a message and how to process it, a set of encoding rules for expressing instances of application-defined datatypes, and a convention for representing remote procedure calls and responses."^[3]

^[3] http://www.w3.org/TR/SOAP/

In its earlier incarnations, the acronym SOAP used to stand for "Simple Object Access Protocol," though that meaning has ceased to exist in the SOAP 1.2 specification. This is undoubtedly a good thing since SOAP isn't especially simple, it's not exclusively designed for object access and it is more a packaging mechanism than a protocol per se.

In the following sections, we examine SOAP in some depth from its basic use pattern and XML document structure, encoding schemes, RPC convention, binding SOAP messages, transport protocols, to using it as the basis for Web services communication.