How can I determine why my ASP.NET application is recycling

Question

I am running an ASP.NET application on an IIS7 server. It has been running fine for a long time, but over the past week or so it has been discarding all users' sessions several times a day. I enabled all of the application pool recycle logging options as described in http://blogs.iis.net/ganekar/archive/2008/12/12/iis-7-0-application-pool-recycles-log-a-event-in-windows-event-log.aspx, but I didn't get anything in my event log.

There are no errors in the event log, and no visible symptoms except that all my users lose their sessions.

Are there any other reasons that IIS would recycle my application pool? Is there any other type of logging that I can enable to find out what is happening?

Answer 1

When you experience this behavior, how recently have you done a deployment of your files to the server?

There is a nasty configuration option called numRecompilesBeforeAppRestart on the compilation tag:

<system.web>
    <compilation debug="true" numRecompilesBeforeAppRestart="15">

http://msdn.microsoft.com/en-us/library/system.web.configuration.compilationsection.numrecompilesbeforeapprestart.aspx

This value defaults to 15. I've been through an application killing all user sessions before and this was the culprit for me. For roughly a day after a lightly used web application was updated (new files copied to the server; this ended up overwriting EVERY file, numbering into the hundreds), we'd get constant AppDomain restarts evidenced by all Session values for all users disappearing.

I found this bug report listing the behavior I'm experiencing: http://support.microsoft.com/kb/319947

Here is the really important relevant text to my scenario:

However, this problem occurs when you load many new .aspx or .ascx files to the server (for example, 61 files). The server unloads the application when the first 15 files are recompiled and every time another 15 files are recompiled until the server reaches 61. This results in four application restarts even though only one is required.

I switched the value to 99999 and the problem went away. This means more memory will build up in my worker process, so I added a daily recycle (3am when my site has no users) to the IIS AppPool recycle settings.