Advanced: How many times does HttpModule Init() method get called during application's life?

Question

Web application initialization is as follows:

1. As we know when IIS receives the first request for a particular Asp.net application resource, IIS creates an instance of a HttpApplication (defined in global.asax codebehind).
2. When this new instance is created it's initialization happens that also checks all configured HTTP modules.
3. All modules are then instantiated and put in the application's Modules collection (of type HttpModuleCollection)
4. modules are looped through and their Init() method is called (when they register for request events)

As far as I understand it the above scenario happens when a web application is started/initialized (hence application start event).

What happens with modules?

Are they (re)instatiated on each request or reused from the Modules property on each consecutive request while the web application is alive? As I understand IIS and Asp.net they are reused through the whole life of a web application.

If they are reused, can we assume that their Init() method is actually a pseudo event handler for application start event? The thing is we can't attach to application level events within http modules. But if they are being reused we could use Init() as application start event and do whatever we'd put in global.asax instead.

Question

Can we assume that module's Init() method is called only on application start event? Could we use this assumption to i.e. register routes for applications whose global.asax codebehind we can't change? web.config is usually accessible and we can change it the way we want.
Would this actually work?

Additional info

We can check HttpApplication code and check its InitModulesCommon() method. This one actually calls Init() of each registered HTTP module. What is more interesting is that this method is only used by InitIntegratedModules() and InitModules() methods. Which are both used only in HttpApplication.InitInternal() method. This is the basis of my assumptions, but I would like to know whether someone has abused IHttpModule.Init() for application start event.

Answer 1

Application_Start is only run once for the lifetime of your application.

IHttpModule.Init is run for each instance of HttpApplication, before request processing begins. See the walkthrough. Init is where you can register events used to process the request.

An instance of HttpApplication can be reused for multiple requests. ASP.Net pools HttpApplication objects, so the Init will be called once for every new instance of HttpApplication

Answer 2

Init() is called only once

After I tested this the inner workings of IHttpModule initialization are as follows:

1. Every IHttpModule is initialized at web application start by instatiating and a call to Init() method
2. HttpApplication stores all module instances in its Modules property
3. Modules are then reused for the whole life of an HttpApplication and are not discarded/reinitialized as long as the application is alive

So the best outcome is

You can't attach a IHttpModule to application level events, but you can use its Init() method as pseudo application start event delegate. Inside it you can execute any code that you'd usually put inside Application_Start delegate in your Global.asax.