In this continuing series covering the basics of Web services, I've described Web services and explained how to format requests to them (as XML) and how to communicate with the services using SOAP. Yet, given that I know a Web service exists, how do I know where on the Web it might be available? How do I know just what sort of message to send? How do I know the format of a SOAP envelope's contents so that the remote server will understand what I want done?

The Web Services Description Language (WSDL) is an XML format for describing Web services. Completed by Microsoft, IBM, and Ariba last September (2000), WSDL form part of IBM's Web Services Toolkit (see RESOURCES), Microsoft's .Net technology, and Sun Microsystems' SunONE Web-services platform.

A WSDL document defines several things to support description of Web services:

• services – information or data processing available somewhere on the Web;
• ports – addresses where a service is available (a particular service may be available at multiple ports);
• messages – describe the structure of the data passed to and returned from services through ports;
• operations – tie messages and method calls together (indicating which methods take which arguments);
• portTypes – group related operations;
• data types – used to define messages, described in XSD (XML Schema);
• bindings – "ground" the abstract data types and structures defined in WSDL to actual data representations.

WSDL is an XML application that models making information services available over the Web in a transport-independent way. WSDL's flexibility comes at the cost of some complexity. Despite a simple underlying idea, WSDL takes a bit of effort to learn. Still, WSDL is poised to become a major enabling technology for the developing "service Web".