Chapter 18. SOAP and RDF

This chapter is about two popular XML applications: the Simple Object Access Protocol (SOAP) and the Resource Description Framework (RDF).

SOAP is an XML-based communications protocol that lets applications communicate over the Web. As applications move to the Web, SOAP is becoming more popular. Web services let applications become distributed, with one part in the client's computer and other parts on the Internet; you can even call procedures across the Internet. Web services and SOAP are a big part of Microsoft's .NET initiative. In fact, it's easy to create Web services in .NET's Visual Studio if you're connected to the Microsoft Internet Information Server (IIS); you just select the Web Service icon for the type of project you want to create and then add the code you want for the procedures that you want client applications to call. Visual Studio uploads the project to the server automatically. Then you create a client application as a new project, add a reference to the Web service you created, and run the client application. It's practically the same as developing your entire application on the same machinebut your application is actually distributed over the Internet and uses SOAP to communicate among the various parts .

SOAP lets applications send data in messages in a platform-independent, language-independent way. It's an easy protocol to understand, as we'll see. SOAP was originally designed so that distributed applications could communicate through corporate firewalls. SOAP sends its XML-based messages using HTTP, which is already well established on the Internet, and that's a large reason for its success: It provides a backbone for distributed applications using existing technologies. You can also send attachments using SOAP messages, and we'll do that here too.

As you can gather from its name , the RDF is a language that lets you describe resources. Although it's typically used to describe Web documents and sites, you can also use it to describe CD collections, books in a library, a collection of antique pen knives, and more. It's a general-purpose XML application that is helping to standardize the way people handle data on the Internet. Some consistent way of working with vast amounts of data on the Internet is sorely needed, and RDF is one possible solution. You may recall our discussion of canonical XML, which puts XML documents into a format that may easily be compared. RDF is actually stored in documents separate from the resources that it describes, and it provides a standard description language.

We'll see both of these XML applications in this chapter. I'll start with SOAP.