The IIS Process Model Features 

A process model is needed to run more than one web-site, web application or web-service securely and reliably on a single machine. In Shared Hosting scenarios hundreds or even thousands of web-sites run on an individual machine. The code running on these web-sites is usually not well tested, if at all. Without a powerful process model the result would be extremely poor reliability. But a process model not only guarantees availability; it also needs to isolate them so that individual web applications don't interfere with each other. 
  
The Windows Process Activation Service (WAS) is the system component providing the Process Model in IIS 7.0. This article discusses the WAS features and functions. 
  
1. Application Pools
  
Processes are the containers that provide isolation and security boundaries in the Windows Operating System. Consequently web applications or web-sites have to run in separate processes in order to achieve isolation between them. In the IIS terminology the management unit for these separate IIS processes is called 'Application Pool'. 
  
An Application Pool can be configured to run a group of URLs, i.e. web-sites, web applications or web-services. When a client requests one of these Application Pool URLs an IIS worker process (w3wp.exe) is spawned by WAS to execute the code necessary to send a response. WAS's main task is to manage Application Pools - WAS spawns worker processes, monitors their health, recycles them if necessary and makes sure none of them consume more resources than specified in the corresponding AppPool configuration. WAS is also the arbiter and collector for run-time and state data, e.g. performance counters, site and Application Pool state. 
  
2. Architectural Diagram 
  
The diagram below shows the core pieces of the IIS7 architecture.
  
       
  • Configuration is stored in the applicationhost.config file (left).  
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  • HTTP.SYS is a kernel-mode component that listens to the network, accepts connections, assigns requests into Application Pool queues and queues these requests if no IIS worker process is running. HTTP.SYS also caches responses and does SSL in IIS 7.0 
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  • The worker process on the right hosts all custom code (e.g. ASP and ASP.Net pages, modules, ISAPI filter and extensions etc.). There can be hundreds of these worker processes depending on how the IIS Administrator decides to isolate his web-sites and applications. IIS worker processes pick up waiting requests from the corresponding HTTP.SYS request queue. 
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  • The Windows Activation Service is a system service that runs in SVCHOST.EXE. WAS reads configuration from applicationhost.config, reacts to configuration changes, manages worker processes, controls their live time and health, recycles them if necessary and prevents resource exhaustion. W3SVC is another service living in the same SVCHOST.EXE as WAS. W3SVC hosts the HTTP specific part of the IIS process model; W3SVC configures HTTP.SYS with the URLs to listen on and is called by HTTP.SYS if requests arrive and worker processes are needed. 
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