Windows Process Activation Service (WAS) is a tool for managing worker processes that contain applications that host Windows Communication Foundation (WCF) services.
Worker processes handle requests that are sent to a Web Server for specific application pools. Each application pool sets boundaries for the applications it contains.
The WAS process model generalizes the IIS 6.0 process model for the HTTP server by removing the dependency on HTTP. This allows WCF services to use both HTTP and non-HTTP protocols in a hosting environment that supports message-based activation and offers the ability to host a large number of applications on a given machine.
WAS was first introduced in Internet Information Services (IIS) 7 and is found in Windows Server 2008 R2. It runs most securely when on .NET Framework 3.5.1 or on IIS 7.0.