Instance Deactivation - an Optimization technique in WCF

Figure 10 Contexts and Instances

Sessions actually correlate the client messages not to the instance, but to the context that hosts it. When the session starts, the host creates a new context. When the session ends, the context is terminated. By default, the lifeline of the context is the same as that of the instance it hosts. However, for optimization purposes, Windows Communication Foundation provides the service designer with the option of separating the two lifelines and deactivating the instance separately from its context. In fact, Windows Communication Foundation also allows the situation of a context that has no instance at all. I call this instance management technique context deactivation. The usual way for controlling context deactivation is through the ReleaseInstanceMode property of the OperationBehavior attribute:

 Instead of making a design-time decision on which methods to use to deactivate the instance, you can make a run-time decision to deactivate the instance after the method returns. You do that by calling the ReleaseServiceInstance method on the instance context. 

Instance deactivation is an optimization technique, and like all such techniques it should be avoided in general cases. Consider using instance deactivation only after failing to meet both your performance and scalability goals and when careful examination and profiling has proven beyond a doubt that using it will improve the situation. If scalability and throughput are your concern, choose the simplicity of the per-call instancing mode, and avoid instance deactivation.

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