title: IIS 8: random "(503) Server Unavailable Server Too Busy" errors
body: <p>I have an IIS server running Windows Server Core 2012 R2. This server will replace another server that we already have and that plays the same role. The server hosts 80 ASP.Net websites and all the sites are in the same application pool. The queue limit is at 1000 (the default).</p>
  
  <p>We have homemade monitoring program that regularly makes a request to the <code>/login.aspx</code> page of each site <strong>sequentially</strong> to check that it's up. </p>
  
  <p>On this new server, the monitoring program regularly encounters <code>(503) Server Unavailable Server Too Busy</code> errors. </p>
  
  <ul>
  <li>When the error occurs, there is nothing logged in the Event log about IIS or ASP.Net errors. </li>
  <li>I know that the worker process does not crash or gets recycled (w3wp keeps the same PID). </li>
  <li>Using Failed Request Tracing I could not find anything of value (the failing requests are logged but I can't find a cause in the log detail). </li>
  <li>Using Perfmon I confirmed that the number of queued request remains very low.</li>
  </ul>
  
  <p>If I modify the monitoring program to sleep 1sec between each site, then I don't get the error anymore. </p>
  
  <p>Any idea?</p>