Most Web sites built with ASP.NET aren't very scalable. They suffer a self-imposed glass ceiling that limits the number of requests they can process per second. These sites scale just fine until traffic rises to the level of this invisible ceiling. Then throughput begins to degrade. Soon after, requests start to fail, usually returning "Server unavailable" errors.
The underlying reason has been discussed many times in MSDN®Magazine. ASP.NET uses threads from a common language runtime (CLR) thread pool to process requests. As long as there are threads available in the thread pool, ASP.NET has no trouble dispatching incoming requests. But once the thread pool becomes saturated-that is, all the threads inside it are busy processing requests and no free threads remain-new requests have to wait for threads to become free. If the logjam becomes severe enough and the queue fills to capacity, ASP.NET throws up its hands and responds with a "Heck no!" to new requests.
