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Outlining What Is Wrong with the Classic Singleton Implementation
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A few things are wrong with the classical Singleton pattern implementation shown in the previous section: A singleton is only a singleton in its own AppDomain. Other AppDomains would need to instantiate their own singleton, causing potential data consistency issues. For example, imagine having to share configuration information in multiple AppDomains. It isn t easily possible to shut down a singleton, and therefore data might be lost. Every class that wants to be a singleton needs to implement a singleton type class structure, requiring changes. To make the class thread-safe, additional code is needed. In general, implementing a singleton using the classical approach isn t that versatile, even though for a confined context it works extremely well. The point to remember is that its scope its limited. Let s see how to implement a versatile singleton.
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Implementing a Versatile Reader-Writer Singleton
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The problem with the classical singleton isn t the Singleton pattern, but the way that it s implemented. The original pattern mixed together multiple techniques that should have been separated. Specifically, what shouldn t have been made part of the singleton implementation is the instantiation and data type reference. A versatile singleton should include the following attributes: A singleton should act as a container and delegate as much functionality as possible to other interfaces. Instantiation is delegated to a factory that allows the instantiation of a class that may exist in the local AppDomain, or, using .NET remoting, on another computer. The singleton only manipulates a local class that may be a proxy. The singleton data should be immutable to increase performance. The singleton implementation should be responsible for calling functionality that verifies the correctness of the immutable singleton data. If the singleton data needs updating, the singleton implementation manages the calling of the external factory. Figure 7-4 is the UML diagram of the versatile singleton implementation.
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Figure 7-4. UML diagram for a versatile singleton implementation As you see in Figure 7-4, the client interacts with two classes, Singleton<> and SingletonData. Singleton<> is a general class that defines all singletons. SingletonData is a user-defined class that will be used in a single instance.
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ISingletonBuilder<> and SingletonBuilder represent the classes used to instantiate SingletonData. The idea is that delegating the instantiation to SingletonBuilder, a local AppDomain singleton could in fact be a proxy to another singleton executing in another AppDomain or on a remote machine. The SingletonBuilder is in complete control. From an execution point of view, the way the versatile singleton considers SingletonData is as immutable. The SingletonData class is updated using external means that the consumer of SingletonData doesn t know about. Internally, Singleton<> implements a verification and update of the SingletonData mechanism. Immutability and Threading The user is responsible for defining a builder that instantiates, verifies, and indicates how the internal Singleton<> update process executes. Following is the definition for the ISingletonBuilder<> interface that the user is responsible for: public interface ISingletonBuilder< type> where type: class { DelegatePredicate< type> IsValid { get; } DelegateTransformer< object, type> NewObject { get ; } int SleepTime { get; } bool KeepPolling { get; } } The interface definition provides no method to directly instantiate the type SingletonData. The approach chosen is to use functors to instantiate and verify the object. Functors were chosen because they make it possible to combine operations and use the Chain of Responsibility pattern. Then whenever a request is made for instantiation of a singleton data object, one transformer in the chain can perform the action. The other properties relate to the execution of the Singleton<> update mechanism. The other methods of the ISingletonBuilder<> interface relate to the topic of updating the data of the singleton. A singleton is a performance bottleneck. You ll have multiple clients accessing the same data, and if the data is updated, then you ll need synchronization. The best strategy when developing singletons is to consider the data mostly read-only. This means the data from the singleton will be read-only for the most part and occasionally modified. If a singleton ends up being read-write, then most likely you don t need a singleton, but a producer-consumer. A type of producer-consumer is implemented in the next section.
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