Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

Figure 12-4: The SalesReport virtual directory created to make the remotable objectaccessible.If you choose IIS as the activation agent, you must be aware of a few things. IIS canlisten only to the http channel; any other channel you indicate is simply ignored. Theway IIS applies the information read from the web.config file is hard-coded and can't beprogrammatically controlled or changed. However, you can create a global.asax file inthe virtual folder, hook the Application_Start event, and then execute some customcode. In addition, the inevitable use of SOAP as the underlying protocol increases theaverage size of network packets.NoteAs often happens, the use of IIS as the activation agent has prosand cons. You don't need to write any extra code, but you lose a bitin flexibility. Regaining the lost flexibility is still possible, but at theprice of writing nontrivial code. For example, you can write anApplication_Start event handler and apply extra binary formatters atboth ends of the http channel. In this way, the SOAP packets willcontain binary data and you'll save some bytes.Using IIS as the activation agent is natural when you plan to expose the same remoteservice through .NET Remoting and Web services. So let's assume in our exampleapplication that IIS is the activation agent and SalesReport is the virtual directory.Activation PoliciesIn addition to the remotable object's identity, channels, and ports, the serverconfiguration file also contains another important piece of information—the objectactivation policy. An MBR remotable object can be either server-activated or clientactivated.Server-activated objects are created by the server only when the clientinvokes the first method through the local proxy. Client-activated objects are created onthe server as soon as the client instantiates the object using either the new operator ormethods of the System.Activator class.In addition, server-activated objects can be declared as Singleton or SingleCall objects.A Singleton object has exactly one instance to serve all possible clients. A SingleCallobject, on the other hand, requires that each incoming call is served by a new instanceof the remotable object. A remotable object declares its required activation policy in theconfiguration file through specific subtrees placed below the node.Server-Side ActivationServer-activated objects are remotable objects whose entire life cycle is directlycontrolled by the host application. Server-activated objects are instantiated on the441