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Client-Dispatcher-Server Pattern
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In 3, you saw a simple plug-in factory. This factory worked, but with the side effect that any assembly that was loaded couldn t be unloaded. The plug-in factory was only capable of dealing with assemblies available on the local computer; it didn t have the capability to execute code on another computer. The simple plug-in factory could only deal with focused requirements. In general, the Factory pattern isn t capable of dealing with complexity, even though it should. Remember, though, that the intent of the Factory pattern is to remove references so that the interface can be separated from the implementation. To make the Factory pattern work in more complex scenarios, another pattern has to be used the Client-Dispatcher-Server pattern.2 This new pattern is similar to the Factory pattern, but can deal with location and resolution issues. The Client-Dispatcher-Server pattern addresses the resolution and location problems by introducing an intermediate that is responsible for establishing a connection between a client and a server. The intermediate is called the dispatcher. The pattern itself, like the Factory pattern, is relatively simple. Where the Client-Dispatcher-Server pattern differs from the Factory pattern is in the implementation. The Client-Dispatcher-Server pattern has more implementation details and is technologically specific, as it could involve network communications.
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2. Frank Buschmann, et al., Pattern-Oriented Software Architecture, Volume 1: A System of Patterns (Indianapolis: Wiley Publishing, 1996), p. 323.
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CHAPTER 4 APPLICATION ARCHITECTURE
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Defining the Client-Dispatcher-Server Pattern Architecture
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The formal architecture of the Client-Dispatcher-Server pattern defines a client, server, and dispatcher. The client requests a server service from the dispatcher, and then uses the provided service. The server registers itself with the dispatcher and provides services to clients. The dispatcher is responsible for cross-referencing a service request from the client to a server. Having found a server, the dispatcher is responsible for instantiating the service. In simple UML terms, the architecture is similar to what is shown in Figure 4-7.
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Figure 4-7. UML architecture of Client-Dispatcher-Server pattern The Client uses both the Dispatcher and the interface IServer. The Server uses the Dispatcher to register itself, and the Dispatcher uses the Server when instantiating the type. The Dispatcher returns to the client the IServer interface. Note that the UML interface means either a .NET interface type or an abstract base class. However, as you ll see in the section Understanding the Big Picture of AppDomains later in this chapter, abstract base classes have major problems. Before tackling the architecture and implementation of the Client-Dispatcher-Server pattern, let s take a look at the two dispatcher types: static dispatchers and dynamic dispatchers.
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A Static Dispatcher Architecture
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A static dispatcher is very similar to a plug-in architecture in that the assembly is loaded at runtime. Let s review the plug-in architecture and then compare it to a static Client-DispatcherServer pattern. Plug-ins are dynamically loaded assemblies that implement a specific interface. In the context of the dispatcher, it means that the servers are plug-ins. The dispatcher manages the plug-ins through some resolution technique that associates an interface instance with a type. For example, if the type ExternalServer were implemented in the assembly ExternalServers.dll, then the dispatcher could be written as follows:
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CHAPTER 4 APPLICATION ARCHITECTURE
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class Dispatcher { public type CreateInstance<type>( string assemblyPath, string @class) { Assembly assembly; assembly = Assembly.Load( AssemblyName.GetAssemblyName( assemblyPath ) ); return (type)assembly.CreateInstance( @class); } } class Client { public void DoSomething() { IExternal obj = CreateInstance<IExternal>( "ExternalServers.dll", "ExternalServer" ); // Do something with obj } } The type Dispatcher has a single method, CreateInstance, which can dynamically load the assembly ExternalServers.dll and instantiate the type ExternalServer. The method Assembly.Load loads an assembly, and the method assembly.CreateInstance instantiates a type in the loaded assembly. The method CreateInstance uses Generics to typecast the Object type to the required type. This solution of dynamically loading and instantiating a type works well, with one ramification: any loaded assembly can t be unloaded if the assembly changes. If the application were a client application in which the assembly doesn t change during execution, then the type Dispatcher is good enough. If the application were executing on the server side, then reloading a plug-in might be an issue. The string identifiers used in the method call CreateInstance<IExternal> are hard-coded. One major additional feature of the Client-Dispatcher-Server pattern is that the dispatcher has to include functionality to perform resolution of an identifier to a type in an assembly. The Dispatcher type in the simple example assumed that the client would know the reference identifiers. The Dispatcher s added value was knowing the name of the path where the assembly and type resided. From a pattern purists point of view, it could be argued that the simple example isn t an implementation of a dispatcher because the server didn t contact and register itself with Dispatcher. That is true, but here is where I tend to be more flexible about the exact behavior of the dispatcher. Having a server contact the dispatcher and then register itself is, in my opinion, one way of implementing a dispatcher. As you ll see demonstrated in later chapters, the Client-Dispatcher-Server pattern can implement and extend other patterns. As a general rule of thumb, a dispatcher is responsible for resolving an identifier to a type and assembly. The resolution of the type and assembly can happen in one of the following ways: Configuration file: A configuration file is loaded and processed by the dispatcher. When a specific type needs to be instantiated, the dispatcher cross-references the requested type with the configuration information. The advantages of this approach are configuration of the application at runtime is possible, and you can use versioning information to load specific versions in specific contexts. The disadvantages of this approach are the administrator needs documentation from the developer on available plug-in implementations, and an improper configuration file can cause havoc when debugging a production issue.
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