Application Interfaces and Web Services

8.4 Application Interfaces and Web Services

We've discussed resource gateways as ways for web servers to communicate with applications. More generally, with web applications providing ever more types of services, it becomes clear that HTTP can be part of a foundation for linking together applications. One of the trickier issues in wiring up applications is negotiating the protocol interface between the two applications so that they can exchange dataoften this is done on an application-by-application basis.

To work together, applications usually need to exchange more complex information with one another than is expressible in HTTP headers. A couple of examples of extending HTTP or layering protocols on top of HTTP in order to exchange customized information are described in Chapter 19. Section 19.1 talks about layering RPCs over HTTP POST messages, and Section 19.2 talks about adding XML to HTTP headers.

The Internet community has developed a set of standards and protocols that allow web applications to talk to each other. These standards are loosely referred to as web services, although the term can mean standalone web applications (building blocks) themselves. The premise of web services is not new, but they are a new mechanism for applications to share information. Web services are built on standard web technologies, such as HTTP.

Web services exchange information using XML over SOAP. The Extensible Markup Language (XML) provides a way to create and interpret customized information about a data object. The Simple Object Access Protocol (SOAP) is a standard for adding XML information to HTTP messages.^[3]

^[3] For more information, see http://www.w3.org/TR/2001/WD-soap12-part0-20011217/. Programming Web Services with SOAP, by Doug Tidwell, James Snell, and Pavel Kulchenko (O'Reilly) is also an excellent source of information on the SOAP protocol.