IIS 6 – Viva the Application Pool

Although IIS 6 application pools are separate EXEs, they are highly optimized for HTTP operations by directly communicating with a kernel mode HTTP.SYS driver. Incoming requests are directly routed to the appropriate application pool. InetInfo acts merely as an Administration and configuration service – most interaction actually occurs directly between HTTP.SYS and the Application Pools, all of which translates into a more stable and higher performance environment over IIS 5. This is especially true for static content and ASP.NET applications.

An IIS 6 application pool also has intrinsic knowledge of ASP.NET and ASP.NET can communicate with new low level APIs that allow direct access to the HTTP Cache APIs which can offload caching from the ASP.NET level directly into the Web Server’s cache. In IIS 6, ISAPI extensions run in the Application Pool worker process. The .NET Runtime also runs in this same process, so communication between the ISAPI extension and the .NET runtime happens in-process which is inherently more efficient than the named pipe interface that IIS 5 must use. Although the IIS hosting models are very different the actual interfaces into managed code are very similar – only the process in getting the request routed varies a bit.