JAX-RPC and SAAJ are enabling technologies that allow companies to create electronic business applications that exchange information using XML and SOAP. WSDL (see Chapter 5) is the means by which a developer finds out about the interface provided by a web service. As you saw in Chapter 6, once you have a WSDL definition for a service, you can quickly create the client-side stubs that an application needs in order to communicate with it using JAX-RPC. The remaining question is, given that your business needs to find a provider for a specific service, how do you go about locating businesses that offer that service, evaluate their offerings, and, if appropriate, fetch the WSDL definition for the service itself? The answer lies in the XML-based registries that are currently being established on the Internet.
A registry contains information that allows businesses to discover and make use of the services of potential corporate electronic partners. A business might want to deal directly with another party (purchasing components or other services from that company), or perhaps, it may intend to add value to the other party's offerings�for example, by providing a specialized interface to the other's web service, such as the one offered by Amazon.com.
A provider company submits an entry to a registry and categorizes it in various ways that will make it easier to find. The submitting company might include in its registry entries links to technical specifications that describe its service at the level required by a potential implementor of an application client, or it might include marketing information for the consumption of management-level decision makers. The enquiring company searches the registry for information using various criteria, such as name hints (thus treating the registry as a source of White Pages information), categories of interest (a Yellow Pages search), or a geographical location (a Green Pages search). The information that it retrieves can be inspected and compared with what is available from other sources. Finally, if a business agreement is reached, the client company retrieves the WSDL definition for the service that it needs to use, which is typically also available through the registry, and uses that as the basis for creating a SAAJ- or JAX-RPC-based application client.
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