ASP.NET Pipeline Architecture

ASP.NET Pipeline Architecture

The HTTP pipeline always processes requests in an instance of the worker processes. By default, there will only be one worker process in use at a time. (If your Web server has multiple CPUs, you can configure the pipeline to use multiple worker processes, one per CPU.) This is a notable change from native IIS, which uses multiple worker processes in order to isolate applications from one another. The pipeline's worker process achieves isolation by using AppDomains. You can think of an AppDomain as a lightweight process within a process. The pipeline sends all HTTP requests targeting the same virtual directory to a single AppDomain. In other words, each virtual directory is treated as a separate application. This is another notable change from native IIS, which allowed multiple virtual directories to be part of the same application.

ASP.NET supports recycling worker processes based on a number of criteria, including age, time spent idle, number of requests serviced, number of requests queued, and amount of physical memory consumed. The global .NET configuration file, machine.config, sets thresholds for these values (see the processModel element). When an instance of aspnet_wp.exe crosses one of these thresholds, aspnet_isapi.dll launches a new instance of the worker process and starts sending it requests. The old instance terminates when it finishes processing pending requests. Recycling of worker processes promotes reliability by killing off processes before their performance begins to degrade from resource leaks or other runtime phenomena.