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WSDL stands for Web Services Description Language, and it is a method of describing a Web Service using XML. For Web Services to work cross platform, they need to have a standard way to transmit and describe what happens within the Web Service. WSDL provides the means to describe what messages and variables exist within the Web Service whereas Universal Discovery, Description, and Integration (UDDI) is a standard way of publishing a Web Service to your desired audience. WSDL is part of the UDDI standard; Chapter 5 covers this in greater detail.
Having a standard way of describing a remote object is not unique to Web Services. CORBA has the Interface Description Language (IDL) that describes the CORBA object to the client, and Microsoft has the Microsoft Interface Description Language (MIDL) for COM+. IDL and, to a lesser extent, MIDL conform to the standard set by the Object Management Group (OMG). The OMG manages certain standards that the industry uses, just like the W3C, where particular aspects of a particular protocol are maintained by a committee and the results are published on the Web.
Figure 4.1 shows how a Web Service client uses a WSDL file. Even though this example is specific to Web Services, this interaction is similar for all remote object technologies.
Figure 4.1: How a client asks for a WSDL file and uses it to create an interface to the methods from the Web Service.
Before any Web Service call is made, the user creating the client uses a tool to grab the WSDL file off the server; this is shown in the first transaction of Figure 4.1. Many Web Service technologies have a standard way of revealing their WSDL file so it isn’t difficult for a user to find.
Once the user has the WSDL file, some technologies, such as Microsoft’s .NET, allow the user to then create a proxy to the Web Service. This proxy then acts as an interface to the Web Service, allowing the client-side code to easily access the Web Service. In the case of Figure 4.1, the WSDL file reveals to the client that the Stock Quote server has three methods available: Get Quote, Active Account, and Get Research. The client now has an interface to each of these methods and is able to use them.
This chapter breaks down each individual section of the WSDL file that describes the GetStockQuote Web Service used in examples in the previous chapters.