SOAP

Simple Object Access Protocol (SOAP) is a simple, extensible mechanism for decentralized information exchange using XML. It can be used to access remote services in a platform-independent, protocol-independent, programming language-independent manner, and is fast becoming a de facto standard for Web Services.

Systems using SOAP interact with each other by sending and receiving SOAP messages. SOAP consists of

• An envelope that defines the contents of a SOAP message, who has to process it, and whether it is mandatory or optional.

• A serialization mechanism that can be used to exchange application data types.

• A convention that can be used for remote procedure calls.

SOAP does not depend upon any network protocol. SOAP data is sent as XML text to enable a standard data representation that can be interpreted by any standard XML parser. Hence SOAP messages can be exchanged via any convenient protocol such as HTTP, SMTP, or FTP. However, HTTP is still the most commonly used protocol today.

Chapter 11, "Web Services Explained," will cover SOAP in detail. It will also show you how to process SOAP requests and responses using Simple APIs for XML (SAX).

SOAP is a very good candidate on which to develop your application because of its extensible and independent nature. However, because SOAP uses XML extensively, applications using SOAP as a transport might be slower than similar ones using HTTP or TCP/IP directly. This is because all data structures will have to be converted into plain text before generating the XML message. At the receiving end, the XML message will have to be parsed to re-create the data structures. This will consume quite a bit of processing power. For the same reason, SOAP might not be suitable if you are developing applications for small devices with constrained environments, such as PDAs and mobile phones.