I've got 2 web servers with mirrored content. There's a load balancer sitting in front of them.

Starting yesterday we've been getting people complaining about 503 errors. i can't find any 503 errors in the IIS log file. However the server host is saying these errors are due to .Net errors in our website which are causing the app pool to recycle.

They pointed out several errors in the windows application event log which look like this:

Log Name:      Application
Source:        ASP.NET 4.0.30319.0
Date:          3/31/2012 8:35:37 PM
Event ID:      1309
Task Category: Web Event
Level:         Warning
Keywords:      Classic
User:          N/A
Computer:      6251.local
Description:
Event code: 3005 
Event message: An unhandled exception has occurred. 
Event time: 3/31/2012 8:35:37 PM 
Event time (UTC): 4/1/2012 1:35:37 AM 
Event ID: e7a580c7b38545cca3416a8595408f24 
Event sequence: 97 
Event occurrence: 1 
Event detail code: 0 

Application information: 
    Application domain: /LM/W3SVC/2/ROOT-1-129777167518960645 
    Trust level: Full 
    Application Virtual Path: / 
    Application Path: C:\inetpub\wwwroot\mywebsite\ 
    Machine name: 6252 

Process information: 
    Process ID: 20000 
    Process name: w3wp.exe 
    Account name: IIS APPPOOL\MyAppPool

In particular they are saying that the account name under Process Information indicates that the app pool is recycling. They said if the app pool were not recycle, the accountname would instead be the folder where the website files are located.

I checked the app pool settings - it's set to recycle every 29 hours. And the rapid fail protection is set to the default of 5 failures in 5 minutes. But i have not seen 5 failures in the event log in that short of a time span.

Can anyone help me confirm if the 503 responses are indeed being generated by the app pools recycling? Or are these errors coming from somewhere else? My guess at the time was their load balancer was the one actually returning the 503 error. But that was just a guess.