This document discusses how I/O threads work in the ASP.NET application model. The worker process uses I/O completion ports and a thread pool to handle asynchronous I/O operations from multiple application domains without blocking threads. When the thread pool is depleted, worker threads wait for pending I/O operations to complete before processing new requests. It also covers the normal and asynchronous page lifecycles in ASP.NET and shows how HTTP modules and handlers process requests.

1. Marcin Daczkowski, AIS.PL

8. I/O Threads
IOCP
Queue
Worker
Threads
Request
Queue
AppDomain #1
AppDomain #2
AppDomain #3
Worker process (aspnet_wp.exe / w3wp.exe)

9. Worker
Threads AppDomain
Database
Web
Service
External Devices
Thread pool
depleted
Worker threads
waiting for I/O
operations to
complete
OtherRequests

10. PreInit
Init
PreRender
PreRenderComplete
Normal page lifecycle
Render
InitComplete
PreLoad
LoadComplete
Load
SaveState
SaveStateComplete
PreInit
Init
PreRender
PreRenderComplete
Render
InitComplete
PreLoad
LoadComplete
Load
SaveState
SaveStateComplete
Async page lifecycle
End
Begin
Async I/O

11.  AddOnPreRenderCompleteAsync

12.  RegisterAsyncTask

13. Session HTTP Module
ASMX
HTTP
Hadle
r
ASP.NET
HTTP
Hadle
r
Cache HTTP Module
Forms Auth HTTP Module
…
…
Custom
HTTP
Hadle
r
DetermineIIS
handler Static
ASP.NET
ISAPI
CGI