ASP.NET and Web Services, Part 1 - What Is a Web Service?
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I'll start by defining a web service by describing what a service is not. For example, a web service is not a subscription service but rather represents some kind of business logic processing on a web server. The key word to understanding "process" is "functionality." It defines what process is. "Web," of course, indicates some kind of event that exposes functionality and occurs on a server residing somewhere on the Internet in response to a client's request. The event requires an event handler (a server-side process that manages client requests by forwarding those requests to the appropriate server where a specified web server exposes its services to interested parties). The event (a web service) is invoked at the behest of a client or some application-based invocation.
For example, International Foreign Currency Exchange (IFCE) submits requests to a service provider such as Reuters for the most recent stock quote. Reuters, herein now defined as a service provider, resides somewhere on the Web on a wide area network (WAN). Because service providers offer many kinds of services, they must reside on a network so they can expose their services to clients on a large scale. The term "service" obfuscates the true meaning of a web service because it contains several different meanings, depending on context.
Viewing a web service from a developer's perspective is one point of view and differs from a user's point of view, or that of a service provider. For example, in J2EE, the name "component" replaces the term service, whereas in Microsoft .NET, web services use "service."
Web services are accessed by calling a listener (a service provider) capable of providing a contract that describes the particular web service a client seeks. The contract specifies both parameters and data types that the listener anticipates receiving, as well as the return type (if any) it sends back to the client. In addition, the listener facilitates incoming requests, and returns responses to clients.
Figure 1 is a visual representation of a service provider's functionality.
Figure 1 A service provider's functionality
 Remember: This is part one of chapter 7 .NET & J2EE Interoperability, by Dwight Peltzer (McGraw-Hill/Osborne, ISBN 0-07-223054-1, 2004). If you like what you see, feel free to click on the following link to get your own copy!
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