Application Pools in IIS

Application Pools is a basic feature of IIS. It uses to isolate the web application from other web application for better security, performance and availability. Here are some basic info about it.


  • It contains more than one web applications that’s why it is called Application pools.
Application Pools in IIS

Worker process:

  • Each pool runs on its worker process ( w3wp.exe) and In-process Session state is stored in the worker process. So, all session state destroyed by stop (manually or accidentally) the worker process.
Application Pools in IIS: Windows Task Manager

Account Identity:

  • Pool associates with account identity. Account identity is a main feature in IIS. Account identity has own .Net framework and user permissions.

Security Levels:

  • There are four built-in account security levels.
    • ApplicationPoolIdentity ( by default )
    • LocalService (used by windows service control manager)
    • LocalSystem (has access to network resources)
    • NetworkService ( buit-in Windows identity and having low privileged )
Built-in account

Custom Account Identity:

  • Custom account identity can be added in application pool. So, it means we can set different account identity on different pools and control the folders/files permissions etc.


  • Each Application Pool load specific .NET CLR version which increase the flexibility.

Queue Length:

  • Each Application Pool has specific queue length if limit exceeded then new HTTP request will receive the 503 “Service Unavailable” message.

Purpose of Application Pools:

  • To associate the application(s) with the desired security levels.
  • To isolate web applications from one another. So, if one application create problem or fail then it would not effect on other web applications.
  • To assign .Net framework versions to different pools.
  • To improve the web application performance.