Programming WCF Services

Instance Deactivation

The sessionful service instance management technique as described so far connects a client (or clients) to a service instance. Yet, the real picture is more complex. Recall from Chapter 1 that each service instance is hosted in a context, as shown in Figure 4-2.

Figure 4-2. Contexts and instances

What sessions actually do is 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 lifetime of the context is the same as that of the instance it hosts. However, for optimization purposes, WCF provides the service designer with the option of separating the two lifetimes and deactivating the instance separately from its context. In fact, WCF also allows the situation of a context that has no instance at all, as shown in Figure 4-2. I call this instance management technique context deactivation. The common way of controlling context deactivation is via the ReleaseInstanceMode property of the OperationBehavior attribute:

public enum ReleaseInstanceMode
{
   None,
   BeforeCall,
   AfterCall,
   BeforeAndAfterCall,
}
[AttributeUsage(AttributeTargets.Method)]
public sealed class OperationBehaviorAttribute : Attribute,...
{
   public ReleaseInstanceMode ReleaseInstanceMode
   {get;set;}
   //More members
}

ReleaseInstanceMode is of the enum ...