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

Figure 12-5: The IIS application mapping table for .rem and .soap URIs.When IIS detects a remoting call, it passes the call to the ad hoc HTTP handlerregistered to handle .soap and .rem resources. Although the object URI gives theimpression of being a URL—that is, a true server-side resource—it is only a name andshould in no way correspond to a physical file. Whether the URI should be a string orthe name of a physical resource depends on the expectations of the handler. Theremoting handler uses .soap and .rem URIs as strings to retrieve the proxy for the type.Singleton ObjectsWhen an object declares itself as a Singleton type, the host application uses only asingle instance of the object to service all incoming calls. So when a call arrives, thehost attempts to locate the running instance of the object. If such an instance exists, therequest for execution is processed. Otherwise, the host creates the unique instance ofthe remote class (using the default constructor) and forwards the request to it.What happens if two requests arrive at the same time? The .NET Remoting subsystemarranges for them to be automatically serviced by distinct threads. This requires thatSingleton objects be thread-safe. Note that this is not a mandatory programming rulebut is more of a practical guideline for real-world scenarios.State management for Singleton objects is certainly possible in theory, but it must becoded in the body of the object in much the same way as you do with Active ServerPages (ASP) and Microsoft ASP.NET pages and even Web services. The idea is thatyou use a shared cache that all clients can access (a sort of ASP.NET Applicationobject), unless you apply a filter on a per-client basis (a sort of ASP.NET Sessionobject).The lifetime of a Singleton well-known object is managed by the .NET Remoting systemthrough a special module called the lease manager (LM). (See the section "MemoryManagement," on page 551, for more information.)444