问题
I was reading the following post How to correctly use IHttpModule *
Now lets think of the word itself. Application pool. Yes pool. It means that a certain web application is running multiple HttpApplication instances in one pool. Yes multiple. Otherwise it wouldn't be called a pool. »How many?« you may ask. That doesn't really matter as long as you know there could be more than one. We trust IIS to do its job. And it obviously does it so well that it made this fact completely transparent for us developers hence not many completely understand its inner workings. We rely on its robustness to provide the service. And it does. Each of these HttpApplication instances in the pool keeps its own list of HTTP modules that it uses with each request it processes.
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I have a question that under what scenario multiple instances of an Application object can run for a single application. Till now I was aware of the fact that a single application object exists per application. So I am curious to know that is this true that multiple instances can run per application and how it is decided ?
回答1:
Each HttpApplication object instance is unique to a single request. If your site is processing multiple requests in parallel, each one must have it's own instance of HttpApplication. That object has per-request state information that must not change during the request's lifetime (including the body of the request and response!)
The instances are pooled, as described in the article. Each one will be reused to service multiple subsequent requests, up to the limit set on the application pool, then it'll be allowed to die off.
Note that you're specifically asking about HttpApplication. This is distinct from the System.Windows.Forms.Application class, which is in fact a singleton class that only exists once per application.
来源：https://stackoverflow.com/questions/6697409/how-many-instances-of-an-application-object-can-run-per-application