The Role of XML Web Services in a Distributed System

XML Web services play much the same role in an application as a component exposed through .NET Remoting. If you're new to XML Web services, you might first think of something like a Web site that enables you to look up the shipping date for a package or an investment site that enables you to get real-time price quotes. However, both of these examples are really entire Web applications. They might rely on a back-end XML Web service to actually retrieve the information, but they also include a user interface in the form of a Web page.

XML Web services, like any other type of middle-tier component, do not provide any type of user interface. They are used exclusively by other applications and are never invoked directly by end users. For example, you might create a package or a stock quote lookup XML Web service and then call this XML Web service from an ASP.NET Web application or a desktop Windows client.

.NET XML Web services are often identified as ASP.NET Web services because they represent an extension of the ASP.NET platform. In ASP.NET, every Web page acts like an individual application that runs briefly on the Web server when requested and returns an XML result over HTTP. ASP.NET is optimized for this task and uses multiple worker threads, automatic process recycling, output caching, and other enterprise-level features to ensure it can handle a large volume of clients. XML Web services work in much the same way as Web pages: They run briefly on the Web server and return a result in an XML markup (generally as a SOAP message). Much as ASP.NET Web pages create the illusion of a continuously running Web application, XML Web services can create the illusion of a stateful object. In reality, however, XML Web services are stateless components that are created and executed just-in-time when a client invokes an XML Web service method. This is one of the most dramatic departures from .NET Remoting, which provides the flexibility to allow singleton and client-activated objects that remain in memory under a set lease lifetime policy.