What are the reason for the application pool to be stopped? I am asking about stopping the application, not recycling.
Does reaching private byte memory limit is one of them? When I debugged my website, it seems that the application pool is keep getting recycled at every minute, then suddenly the application pool is stopped and giving me
Error 503 unavailable.
I found out that actually my hosting provider degraded my private byte memory limit from 500MB into 100MB and without telling me. My application has minimum requirements of 150MB-200MB private bytes limit.
I am here asking this to make sure that the problem was because the memory limit, not the other thing.
Best Answer
It could be that the user identity is outdated, especially if you've tried starting a stopped app pool and the next request again fails.
In IIS, go to the Application Pools under the Server, then find the correct Application Pool for your web site, and click on it. On the Advanced Settings menu to the right, select Identity and change it and enter new user and password. Click on your Application Pool again, and select Recycle to restart it.
You can also try looking at the error message in Event Viewer, under Windows Logs, Application, Details tab.
Taken from HTTP Error 503, the service is unavailable.