Object Activation and Contexts in .NET framework

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The preceding chapter provided an overview of distributed applications and the services that are offered with NET Enterprise Services In this chapter, you learn about the base technology for serviced components This chapter opens by discussion contexts, from both a NET and a COM+ perspective, and then the focus shifts to creating and using a serviced component The chapter then discusses object and application activation, with an emphasis on just-in-time activation and object pooling Specifically, this chapter covers the following topics:
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NET and COM+ contexts Creating a serviced component with the base classServicedComponent and using the required attributes Object activation features with construction strings, just-in-time activation, object pooling, and private components Application activation features to compare library and server applications, Windows services, application pooling, and application recycling
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Contexts
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All code executes in some platform-provided runtime environment In traditional operating systems, the primary form of the runtime environment is a process An operating system process not only provides resources to your code such as virtual memory, threads, and kernel objects, but it also introduces an isolation boundary between your code and everyone else's code This isolation not only provides some degree of protection from other people's faults, but it also provides your code with distinct characteristics such as a unique security principal or current directory Isolating code is not only useful with the process, but also within the process COM, COM+, and the Common Language Runtime (CLR) have all found it necessary to replicate this model to give code finer-grained isolation within a single operating system process In both COM+ and the CLR, the finest-grained unit of isolation is called a context [1] COM+ contexts are a subdivision of a COM apartment, which itself is a subdivision of an operating system process used to group objects to threads CLR contexts are a subdivision of a CLR application domain, which itself is a subdivision of an operating system process used to group objects that share a common security and version policy Contexts are the heart of the COM+ and NET Enterprise Services programming model and architecture Each component that is configured in the COM+ catalog is automatically associated with a context This context enables interception and thus all the services discussed in 1, "Introducing NET Enterprise Services" With COM+, a configured object is always running inside a context The context needed by the object is defined by context attributes that are specified with the serviced component class The context attributes define the runtime requirements of a component If an object is called from a client object that is already running inside a context (because it is a configured object, too), it is checked whether this context is compatible with the context that is required by the object If it is, the object runs inside the same context as the calling object If the context is not compatible with the requirements defined by the component, a new object context is created Both the CLR and COM+ rely on proxies to enforce context boundaries between objects A proxy is an object that acts as a stand-in for the real object that resides in a distinct context The proxy is responsible for switching contexts before and after a method call As a context is defined by a set of properties that state the runtime needs of an object, defining a context for an object means that the object can only be accessed directly from within this context If a client object accessing this object has a different context associated, it cannot invoke methods of the context-bound object directly, and needs a proxy instead Figure 2-1 shows an example of such a scenario Object A defines that it needs a context with the property Synchronization=Not Supported, whereas object B has the need for Synchronization=Required If object A wants to invoke methods with object B, it cannot do so directly; it must use a proxy instead The proxy invokes intermediate code to fulfill the requirement of object B that is running in context Y (for example, by placing a lock)
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