Scaling the Web

T raditionally, access to services offered over the World Wide Web has relied on the interaction between a browser and a Web server using the HTTP protocol. More recently, programmatic access to services over the Web, called Web services, has been the subject of intense activity and standardization efforts.1-2 In this new model, Web service providers use the Web Services Description Language (WSDL)3 to describe the services they provide and how to invoke them. The service providers then register their services in a public service registry using universal description, discovery, and integration (UDDI).4 Application programs discover services in the registry and obtain a URL for the WSDL file that describes the service. Then, the applications can invoke the services using the XML-based simple object access protocol (SOAP) in either asynchronous messaging or remote procedure call (RPC) mode.1,5 Figure 1 illustrates the concept of Web services and how the model differs from that of traditional access to a Web site. Traditional Web sites (Figure 1a) implement all components needed to carry out user transactions: user interface and navigation management, business logic, and access to persistent storage. Web services sites give users access to some or all of these services through programs that provide these services over the Web, as Figure 1b shows. In this case, the travel site uses Web services applications for airline booking, hotel reservation, and car rental reservation.