Maybe this helps
http://support.microsoft.com/kb/307985/en-us/
An HttpApplication class provides a number of events with which modules can synchronize. The following events are available for modules to synchronize with on each request. These events are listed in sequential order:
- BeginRequest
- AuthenticateRequest
- AuthorizeRequest
- ResolveRequestCache
- AcquireRequestState
- PreRequestHandlerExecute
- PostRequestHandlerExecute
- ReleaseRequestState
- UpdateRequestCache
- EndRequest
The following events are available for modules to synchronize with for each request transmission. The order of these events is non-deterministic.
- PreSendRequestHeaders
- PreSendRequestContent
- Error
See the article for full info.