Instance management is my name for the set of techniques WCF uses to bind client requests to service instances, governing which service instance handles which client request, and when. Instance management is necessary because of the extent to which applications differ in their needs for scalability, performance, throughput, durability, transactions, and queued calls—there simply isn’t a one-size-fits-all solution. However, there are a few canonical instance management techniques that are applicable across the range of applications, thus enabling a wide variety of scenarios and programming models. These techniques are the subject of this chapter, and understanding them is essential to developing scalable and consistent applications. WCF supports three types of instance activation: per-call services allocate (and destroy) a new service instance for each client request; sessionful services allocate a service instance for each client connection; and singleton services share the same service instance for all clients, across all connections and activations. This chapter provides the rationale for each of these instance management modes, and offers guidelines on when and how to best use them. It also addresses some related topics, such as behaviors, contexts, demarcating operations, instance deactivation, durable services, and throttling.[1]
[1] This chapter contains excerpts from my articles “WCF Essentials: Discover Mighty Instance Management Techniques for Developing WCF Apps” (MSDN Magazine, June 2006) and “Managing State with Durable Services” (MSDN Magazine, October 2008).
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