How many instances of a web application are running in one worker process?

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I have a single ASP.NET MVC 4 web application (MvcApplication which extends System.Web.HttpApplication) hosted on IIS 8. My application is running on a separate application pool in integrated pipeline mode. Maximum worker process count for this application pool is set to 1.

I understand, that application instances might get pooled and there might be multiple parallel threads, which will call Begin_Request/End_Request for the same instance of my application (AppDomain). Also I understand that if I set "Maximum worker process count" to more than 1 then there will definitely more than one instance of the application (AppDomain).

What I don't understand is how many calls to Application_Start will be issued and how many instances of my static variables (stored in MvcApplication and also there are some singletons, like NHibernate session factory etc.) will exist at a given moment.

Currently I have some problems with cleaning up .NET MemoryCache. Some users of my website are receiving old values from the cache, thus leading me to think that there might exist more than one instance of my global MemoryCache, even when I set "Maximum worker process count" to 1.

What is the maximal number of instances of my application static variables and singletons which will be run in parallel in a single worker process?

Is there any utility or script which would help me to enumerate all AppDomains on IIS?

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asp.netiisappdomainstatic-membersworker-process

1 Answers

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I'm not sure if this applies to you, but HttpModule Initialize methods are called more than once because even though there is only one Application instance. One HttpApplication can spawn multiple HttpModule instances, so make sure those initializations are thread safe.

Application_Start and _End will fire only once per site start and stop with your current settings, so that will not be the issue. If you are using the HttpContext.Cache object, you might have the issue mentioned in this answer:

HashTable in HttpContext.Current.Cache

If you want, you can try using WinDbg and get a dump of what is going on in your process but that is a really heavy-handed tactic that probably won't shed as much light as just looking through the code. Not until you look at it a little harder, and are still stumped IMO.