19.4. Service discovery One of the more advanced ideas underpinning Web services is that of service discovery. At present, humans generally decide with whom a program should share information. The purchasing agent for Miracle Cleanser tells his computer that the order for the "free if you act now" scrub brushes should be sent to the High-on-the-Hog Bristle Company. Some believe that with Web services, that won't be necessary. The purchasing program can search for a supplier using an elaborate registry system called Universal Description, Discovery, and Integration (UDDI).[5] A UDDI registry contains three classes of information: [5] Given the problems that even humans have in evaluating service offerings and providers plus the many intangibles often involved in a decision some some skepticism may be warranted. Full realization of service discovery would seem to require both artificial intelligence from the computer and genuine faith from the humans! white pages They contain general contact information about organizations. yellow pages These list organizations by business category or location. green pages These include the technical aspects of conducting business, including Web service descriptions and schemas. The green page Web Service descriptions are expressed in XML conforming to the Web Services Description Language (WSDL). A WSDL service description in turn affects the structure of the SOAP messages that are used in conjunction with the service. It indicates the operations that the service performs, the message structure for requesting each operation, and the message structures that the operation returns. This dynamic interaction between SOAP and WSDL is what makes this model of service discovery possible. (We discuss UDDI and WSDL in detail in Chapter 23, "Web services technologies", on page 484.) |