ASP.NET Application Level vs. Session Level and Global.asax...confused - asp.net

Question

The following text is from the book I'm reading, 'MCTS Self-Paced Training Kit (Exam 70-515) Web Applications Development with ASP.NET 4". It gives the rundown of the Application Life Cycle.
A user first makes a request for a page in your site.
The request is routed to the processing pipeline, which forwards it to the ASP.NET runtime.
The ASP.NET runtime creates an instance of the ApplicationManager class; this class instance represents the .NET framework domain that will be used to execute requests for your application. An application domain isolates global variables from other applications and allows each application to load and unload separately, as required.
After the application domain has been created, an instance of the HostingEnvironment class is created. This class provides access to items inside the hosting environment, such as directory folders.
ASP.NET creates instances of the core objects that will be used to process the request. This includes HttpContext, HttpRequest, and HttpResponse objects.
ASP.NET creates an instance of the HttpApplication class (or an instance is reused). This class is also the base class for a site’s Global.asax file. You can use this class to trap events that happen when your application starts or stops. When ASP.NET creates an instance of HttpApplication, it also creates the modules configured for the application, such as the SessionStateModule.
Finally, ASP.NET processes request through the HttpApplication pipleline. This pipeline also includes a set of events for validating requests, mapping URLs, accessing the cache, and more.
The book then demonstrated an example of using the Global.asax file:
<script runat="server">
void Application_Start(object sender, EventArgs e)
{
Application["UsersOnline"] = 0;
}
void Session_Start(object sender, EventArgs e)
{
Application.Lock();
Application["UsersOnline"] = (int)Application["UsersOnline"] + 1;
Application.UnLock();
}
void Session_End(object sender, EventArgs e)
{
Application.Lock();
Application["UsersOnline"] = (int)Application["UsersOnline"] - 1;
Application.UnLock();
}
</script>
When does an application start? Whats the difference between session and application level? I'm rather confused on how this is managed. I thought that Application level classes "sat on top of" an AppDomain object, and the AppDomain contained information specific to that Session for that user. Could someone please explain how IIS manages Applicaiton level classes, and how an HttpApplication class sits under an AppDomain? Anything is appreciated.

Answer 1

Nothing lives outside an AppDomain so of course the HttpApplication has to be instantiated inside one.
Step 3 to 6 only happens ONCE in the lifetime of your application. When an ApplicationManager instance has been created it wont be created again for the next request. The same is for HostingEnvironment and HttpApplication. This means that values stored in the Application-collection will be remain there to get for all later requests during the lifetime of the application.
There is one AppDomain per application, not per session or per request.