How to tell why an IIS application pool is recycled

application-poolasp.netasp.net-mvc-3iisiis-7

Background:

I've deployed an ASP.NET MVC 3 app that works on my machine to a shared hosting provider and am discovering some problems that appear to be related to the application pool being recycled. The host has configured recycling to occur under any of these circumstances:

  • Memory usage exceeds 200MB
  • CPU usage exceeds 75% (presumably over a sustained period)
  • 20 minutes of idle time

The restrictions are more relaxed on my development machine so I wasn't seeing recycling like this during development. I don't have admin access to the shared hosting box (understandably) so I can't read the event log to see why this recycling is occurring.

Question:

Is there a way I can find out why my app was recycled (in Application_End for example) so that I can log it to help my debugging?

Best Answer

Without access to the event logs (because you're in a shared hosting environment) the most information you're going to get is from the Application_End event and by asking the HttpRuntime (via reflection) for the values of one or two private members that are sadly not exposed publicly.

To do this add the following code to your Application_End event:

BindingFlags staticFlags = 
    BindingFlags.NonPublic | BindingFlags.Static | BindingFlags.GetField;
BindingFlags instanceFlags = 
    BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.GetField;

HttpRuntime runtime = (HttpRuntime)typeof(System.Web.HttpRuntime)
                        .InvokeMember("_theRuntime", staticFlags, null, null, null);
if(runtime != null) 
{
    string shutDownMessage = (string)runtime.GetType()
         .InvokeMember("_shutDownMessage", instanceFlags, null, runtime, null);

    string shutDownStack = (string)runtime.GetType()
         .InvokeMember("_shutDownStack", instanceFlags, null, runtime, null);

    // Log shutDownMessage & shutDownStack somewhere
}

If I shutdown or recycle my app's application pool I see the following:

HostingEnvironment initiated shutdown
HostingEnvironment caused shutdown -    
   at System.Environment.GetStackTrace(Exception e, Boolean needFileInfo)
   at System.Environment.get_StackTrace()
   at System.Web.Hosting.HostingEnvironment.InitiateShutdownInternal()
   at System.Web.Hosting.HostingEnvironment.InitiateShutdownWithoutDemand()
   at System.Web.Hosting.PipelineRuntime.StopProcessing()

That's probably about as good as it gets.

Update:

I couldn't remember where I found this code but Drew helpfully reminded me it was from a Scott Guthrie blog post.

There are some other private members that could be useful such as:

private ApplicationShutdownReason _shutdownReason;

You can examine these fields in .NET Reflector (if you still have a copy that isn't time-bombed) or one of the alternatives (Open Source Alternatives to Reflector?).

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