The next thing to understand is that the WF API also consists of a runtime engine to load, execute, unload, and in other ways manipulate a workflow which you have defined. The WF runtime engine can be hosted within any .NET application domain; however, be aware that a single application domain can only have one running instance of the WF engine.
Recall from Chapter 16 that an AppDomain is a partition within a Windows process that plays host to a .NET application and any external code libraries. As such, the WF engine can be embedded within a simple console program, a GUI desktop application (Windows Forms or WPF), or exposed from a Windows Communication Foundation (WCF) service.
The WCF Workflow Service Application ...