What is the relationship between Application Pools and worker process threads?

Question

I'm troubleshooting restarts in an ASP.NET application. The application is restarting about 20 times a day. We strongly suspect one part of the application because the restarts began when this particular feature when into production. I've added some logging to those pages using the log4net library, but I'm having trouble interpreting the logs.

When an ASP.NET application is running in an Application Pool, does only a single instance of that application run, or will multiple instances of that application run? I know several worker process threads will be spawned. What is the relationship of the worker process threads to the application(s) running in the App Pool?

I am thinking that I may not be interpreting the results correctly if there are multiple applications logging to the same log. If one is restarted but the other is not, the logs aren't really telling me much about what was happening when the restart occurred.

UPDATE 1

Looking at this page, I find the following information:

An application pool defines a group of one or more worker processes, configured with common settings that serve requests to one or more applications that are assigned to that application pool. Because application pools allow a set of Web applications to share one or more similarly configured worker processes, they provide a convenient way to isolate a set of Web applications from other Web applications on the server computer. Process boundaries separate each worker process; therefore, application problems in one application pool do not affect Web sites or applications in other application pools.

But I am still confused. I know from experience that I can assign two entirely different applications to use the same App Pool. Does that mean that exactly two worker processes will be spawned? Or can there be multiple worker processes spawned for the first app, and multiple worker processes spawned for the second app? If a problem happens in one worker process, can it take down every application running in that App Pool?

UPDATE 2

From this page about using WinDbg, I found this information (emphasis mine):

In IIS 5.x, there is only one Aspnet_wp.exe worker process and one debugger thread. Consequently, only one debugger can be attached to the Aspnet_wp.exe process at a time. This can pose a problem if you're dealing with multiple Web applications on the same machine. In IIS 6.0, you can coerce an AppDomain to run in a separate application pool. (For more information, see "IIS 5.x Process Model" and "IIS 6.0 Process Model" in Chapter 1.) Separate application pools provide multiple W3wp.exe processes. Multiple debugger threads are created in these processes (one in each), allowing you to debug more efficiently.

This sounds to me like each App Pool gets one w3wp.exe process. Am I interpreting that right? And if so, does that still apply in IIS 7.5?

Answer 1

Yes, each application pool is typically a single process¹, but can contain multiple threads. You can assign multiple sites to an application pool, and those sites will all run under the same process, however they will run under different "app domains", which are security contexts that separate the code of one site from another, even if they're running on the same app pool.

Two users hitting the site at the same time can run on different threads, meaning they can run concurrently. That means any logging can have values interspersed. You might want to add a session value to your logging so you can sort based on session.

App pool restarts (recycling) are normal, 20 restarts in a day does not seem unusual. They can happen multiple times per day, and IIS controls when app pools are restarted. It does this whenever it feels it needs to clean up the pool.² Your applications should be written in such a way as to recover gracefully from this (ie, do not keep anything in session that cannot be easily recreated if the app pool restarts).

The app pool can also restart when an unhandled exception occurs in your app. In that case, you want to address the cause of this. Such exceptions are usually logged in the event log.

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^{1 – While you can configure an application pool to have multiple worker processes (this is known as a Web Garden), this is not a typical (nor generally recommended) configuration in my experience.}

^{2 – Note that using IIS Manager you can configure an application to log recycle events to the Windows Event Log. You can also use IIS Manager to set the threshold for when several of the different types of recycle events occur.}