Symptom:

You receive an event similar to this in your DNN event log:

Application Shutting Down Shutdown Details: The AppDomain shut down because of the hosting env ...
Shutdown Details: The AppDomain shut down because of the hosting environment.
Server Name: WEB20

Solution:

• On versions of Plesk before 8.2, the application pool idle timeout was set to 20 minutes.  On Plesk 8.2, SWSoft/Parallels (the makers of Plesk) decided to implement a default of 5 minutes on the application pool idle timeout.  As a result, it's possible if no one is on your site for longer than 5 minutes (or 20, depending on the plesk version), then that idle timeout can trigger, shutting down the app pool.  For an ASP or PHP based site, this really causes no performance impact.  However for a .NET based site (including DNN), DNN will log this event in the DNN event log... and the process of spinning up another application pool/worker process instance can will result in a significantly slow page load for the very next visitor.  This is described in more detail in the section "Slow initial page load" in kb article: http://knowledge.3essentials.com/web-hosting/article/344/Performance-problems-and-tuning-for-ASP.net-based-sites-including-DNN.html.
• To address this, 3Essentials will gladly disable the Application Pool Idle Timeout value on your Application Pool for any customers whose plans include ASP.net at no additional charge.  Simply submit a support request asking for this to be disabled and specify your domain.  When you request this, we'll also disable the default "daily recycle" of the worker process (unless you specify otherwise).  This is a regular recycling of the application pool designed to recycle the application pool once every 27 hours to help keep the process healthy (mostly intended to clear memory usage by the process in event your code or a module within your code has a memory leak).  You can request we do not disable this, or that we set it to a different schedule - just specify your requirements in your support request.