Think of XML Web Services as code that runs across a network, the Internet being the best example of a network. Several technologies make XML Web Services work. As you might imagine, you will need to find Web Services produced by others. You will need to determine what capabilities and information those Web Services provide, and you will need to incorporate those Web Services into your applications so that you can use the capabilities offered . Several acronyms surrounding Web Services may create some initial confusion, but these acronyms do represent technologies that play valuable roles.
Assuming you want to find Web Services offered by third parties, you will need the technology called Universal Description, Discovery, and Integration (UDDI). Think of UDDI as the yellow pages for Web Services. Just as there are several yellow pages providers for telephone numbers in various regions , there are "yellow pages" providers for Web Services. Generally, when you are looking for Web Services, you will be looking for host computers that have UDDI servers. For example, Microsoft has a UDDI server at http://uddi.microsoft.com.
A direct way to use UDDI is to select ProjectAdd Web Reference from the Visual Studio .NET toolbar. This will open the Add Web Reference dialog shown in Figure 13.1. When you enter a host URL in the Address combobox, the dialog will explore the Web site for files ending in .disco . These discovery (DISCO) files ”another acronym associated with Web Services ”contain XML-formatted information that points to a contract reference file. In short, the .disco file points to a .asmx file that represents the Web Service.
Figure 13.1. The Add Web Reference dialog, which uses UDDI technology to look for .disco files that point to Web Services.
When you add a reference to an existing Web Service, the next technology, Web Services Description Language (WSDL, pronounced whiz-dal ), kicks in. The next section demonstrates how WSDL is used to help consume Web Services.