Web Services

Web services represent an important evolutionary step in the building of distributed applications. At present, older systems are isolated and often incompatible blocks. This makes business-to-business or business-to-client integration almost impossible because there are a lot of technological walls between them. This is where the importance of a common technology arises. Web services can make older systems available on the Web, reusing existing software and opening the doors to systems integration. This section provides an overview of the Web services technology and explains how to incorporate it into your newly upgraded application.

There is not a universal definition of a Web service or a minimum standard for the requirements and services offered by one. What is commonly accepted is that a Web service is a software system that is designed to support interactions between different computers in a network.

Communication that takes place between Web service applications and components is encoded with an XML format. Because XML is an open standard that is supported on several platforms, Web services are not platform specific and can be used to communicate between applications that are based in different languages and platforms. For example Visual Basic .NET applications can communicate with Java applications and Windows applications can communicate with UNIX applications.

Additional features of a Web service include:

• A publicly exposed interface with all of the members that can be accessed by a client application or component. This interface can be described in a standard language by using an XML format. The Web Service Description Language (WSDL) is the most widely accepted language for this purpose.

• A mechanism to publish the services offered by a component that can be located by parties who are interested in the service provided. The most accepted directory of Web services that are currently available is Universal Description, Discovery, and Integration (UDDI).

One of the most important innovations in Web services is the use of XML as a transport medium for remote procedure calls. Previous technologies, such as traditional Remote Procedure Calls (RPC), used HTTP for such a purpose. By using XML as a standard, different applications can easily communicate data and service invocations. For an example of a publicly available Web service, see "Microsoft MapPoint Web Service" on the Microsoft Web site.

The .NET Framework Class Library includes the WebService class that provides access to ASP.NET Web service objects. This class can serve as the base class for XML Web services. For more information about the WebService class, see "WebService Class" on MSDN.

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