XML Web Services Overview

XML Web services extend component-based programming to the Internet. They provide a mechanism for packaging data and functionality for reuse by a wide variety of client applications distributed across the Internet. Traditional Web applications are geared toward presenting information in a human-readable format. XML Web services allow Web applications to share this same information with other applications. Doing so enables you to reformat and combine this information with other data to provide a more meaningful presentation.

For example, one of the early Web services to be made available publicly was the Microsoft TerraServer .NET Web Service, also referred to as simply Terra ­Service and available at http://www.terraservice.net. TerraService provides access to a vast database of aerial and topographical images of locations around the world. Developers can access this Web service in their applications, specify location information such as latitude and longitude, and retrieve images that they can display in their own applications.

Web services are built upon a number of Internet standards. This enables interoperability between servers and clients , even if they are executing on heterogeneous systems. The common substrate of these standards is XML, which is platform independent, easy to use, and widespread in its use. The data transferred between a client application and the Web service is marshaled in the form of SOAP messages. Note that some limited form of data transfer can also take place if you use HTTP GET and POST, but SOAP is the preferred and most capable format for packaging data. The Web service documentation ”a description of its API and the format of the SOAP messages used to transfer data between the two endpoints ”is provided by using Web Service Description Language (WSDL). Both SOAP messages and WSDL documents are XML documents whose structure can be described by using XML Schema Definition (XSD) files.