Summary

.NET Framework 2.0 is the first version in which the CLR needs to provide a solution for hosts whose reliability model requires long process lifetimes. These requirements were primarily driven by the integration of the CLR into SQL Server 2005 and the anticipation of other hosts with similar requirements in the future. Before .NET Framework 2.0, CLR hosts that required high availability used a model based on process recycling. This model worked great for hosts such as ASP.NET in which virtually no per-process state was kept, so it really didn't matter which process was used to process a particular incoming request.

To support hosts for which a process recycling model is inadequate, the .NET Framework 2.0 version of the CLR introduces a reliability model based on application domain recycling. In this model, the CLR guarantees that an application domain can be unloaded from a process without leaking any resources. If the integrity of an application domain becomes questionable because of an exceptional condition such as the failure to allocate a resource, the application domain can simply be removed from the process without affecting the integrity of the process. One of the technical challenges of the application domain recycling model is to provide the guarantee that handles to native resources, such as files and kernel objects held by managed code, can always be freed when an application domain is unloaded. Providing this guarantee requires the introduction of new infrastructure in the form of critical finalization, SafeHandles, and constrained execution regions. These three new concepts work together to guarantee that the code required to free a handle to a native resource will always be run, regardless of how quickly an application domain must be unloaded.

Another critical piece of the CLR's design to support long process lifetimes is the notion of escalation policy. CLR hosts use the hosting interfaces to specify a set of rules known as escalation policy that dictate the actions taken by the CLR in the face of various exceptional conditions. Hosts can tailor these actions to their specific requirements to guarantee process integrity.

As you'll see in the next chapter, hosts can use another new .NET Framework 2.0 feature called host protection to restrict the set of APIs an add-in can use, thereby reducing the possibility of having to unload an application domain to maintain process integrity at all.