FIGURE 28-2 The working of a Web service

I will now discuss the working of a Web service in detail. When a client application sends a request for a service, you may need to pass arguments. To pass arguments over the network, the arguments are packaged as a SOAP message and passed to the Web method by using a network protocol. You will learn about SOAP in detail later in this chapter.

Then, the Web service decodes the SOAP message to retrieve the arguments passed to the Web method. Once the arguments are passed to the Web method, the method is executed and the return value is passed to the Web client application.

Having learned about the working of a Web service, you can look at the technologies that are used by a Web service.

Technologies Used in Web Services

You can create Web services by using any language provided by the .NET Framework, such as Visual C# .NET, Visual Basic .NET, and Visual C++ .NET. However, for an application to be able to access a Web service, the client application needs to meet certain requirements. These requirements include a standard

626 Project 5 CREATING A WEB PORTAL FOR A BOOKSTORE

format for describing Web services, a standard format for representing data transfer, and a standard for sending methods and the results returned by the methods across the network. In addition, to be able to access a Web service, the Web client application needs to identify a method for locating the Web service and passing inputs to the Web methods.

As a solution to these requirements, technologies such as XML, WSDL, and SOAP were developed. The following sections discuss these technologies in detail.

XML in a Web Service

XML is a markup language used to describe data in a particular format.This data can be accessed by any application built on any platform. XML allows you to transfer data in a format that is independent of the platform. Therefore, XML is a widely used technology that transfers data across the Internet applications. XML documents store data in the form of text. This makes the XML document easily understood by applications built on different platforms. Moreover, content stored in an XML document is easily transferred over the network.

Having discussed XML in general, you can see how a Web service uses XML. When a Web service client application calls a Web service, the client application passes arguments to the Web method.The Web service processes the Web methods and returns a result to the client application. Because the client application can be built using any platform, the data returned by the Web service is in the form of XML.

WSDL in a Web Service

WSDL is a markup language that defines a Web service. WSDL is an XML file that contains information about a Web service. This information includes the Web services called by a Web site, the methods included in each of the Web services, and the parameters that you need to pass to the Web methods. In addition, WSDL includes information about the results returned when a request is processed by a Web service. For example, WSDL defines the type of the values returned by a Web method.Therefore, WSDL is a vocabulary defined for the creation of a Web service that the developer may need to use while creating a Web service.

In addition to storing information about the Web methods, WSDL stores information about the format used by a user to access a Web service and specifies the location at which the Web service is available. Therefore, WSDL describes the entire mechanism involved in the transfer of data from a Web service client to the Web service and vice versa.

For example, a Web service client application needs to call a Web method that validates the username and password entered by the user. The Web method is created in a Web service. To call this Web method, the Web service client sends a request to the Web service. The request that is sent to the Web method is specified by WSDL.The request is sent to the Web service in the form of XML messages. In this case, WSDL stores the format in which the request is sent.

In addition, when a Web method is called, you need to pass the username and password as parameters. The information about the type and the format of the parameters is stored in a WSDL file. When the request is processed and the result is returned, WSDL stores the format and other information about the results returned.

SOAP in a Web Service

To transfer data from a Web service client to a Web service and vice versa, the transfer protocol used is SOAP. SOAP is a protocol based on XML that is used by a client application to access a Web service. In addition to XML, SOAP uses HTTP for the transfer of data. When a client sends a request, the request is in the form of a SOAP message. The SOAP message also includes the parameters and the method call statement. Based on this information in the SOAP message, the appropriate Web method is called.

As discussed earlier, SOAP is a standard protocol used for communication between a Web service client and a Web service. However, SOAP does not define syntax to be followed while transferring data. Instead, SOAP provides a mechanism for packaging data to be transferred across a network. In addition, SOAP is a transfer protocol based on simple Internet standards. The transfer of data using SOAP takes place in the form of a SOAP package. A SOAP package includes an envelope that encapsulates the data to be exchanged.

In addition to these technologies, Web services uses UDDI to identify the Web services provided by various Web service providers. I will now discuss UDDI in detail.