When does Sitecore run the initialize pipeline? - sitecore

Question

I have been peeking through the Sitecore.Kernel but can't seem to find the place where Sitecore runs the Initialize pipeline. Does it run on every page request or only once on application start? Can you point me to the exact place where the Run method is being called for this pipeline?
update
There reason I ended up asking this question is because I was tracing back, in Sitecore's code, the execution of the mvc.requestBegin pipeline. This is what I found:
the Initialize pipeline runs the InitializeRoutes processor...
Sitecore.Mvc.Pipelines.Loader.InitializeRoutes.Process(PipelineArgs args)
calls...
Sitecore.Mvc.Pipelines.Loader.InitializeRoutes.RegisterRoutes(RouteCollection routes, PipelineArgs args) calls...
Sitecore.Mvc.Pipelines.Loader.InitializeRoutes.SetRouteHandlers(RouteCollection routes, PipelineArgs args) creates new RouteHandlerWrapper objects
here things get a bit fuzzy...
Sitecore.Mvc.Routing.RouteHandlerWrapper.GetHttpHandler(RequestContext requestContext) returns (IHttpHandler) new RouteHttpHandler
and then, finally...
Sitecore.Mvc.Routing.RouteHttpHandler.ProcessRequest(HttpContext context) calls...
Sitecore.Mvc.Routing.RouteHttpHandler.BeginRequest() runs mvc.requestBegin pipeline

Answer 1

I decided to try adding some debugging to a clean installation of Sitecore 7.2.
I created the following simple class:
using Sitecore.Diagnostics;
using Sitecore.Pipelines;
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Web;
namespace SCMVC72.Pipelines
{
public class DebugStackTrace
{
public void Process(PipelineArgs args)
{
StackTrace st = new StackTrace();
Log.Info(string.Empty, (object)this);
Log.Info("*********************************************************************", (object)this);
Log.Info("Debug StackTrace", (object)this);
Log.Info(st.ToString(), (object)this);
Log.Info("*********************************************************************", (object)this);
}
}
}
And the I added the following config file in the App_Config\Include folder:
<configuration xmlns:patch="http://www.sitecore.net/xmlconfig/">
<sitecore>
<pipelines>
<initialize>
<processor type="SCMVC72.Pipelines.DebugStackTrace, SCMVC72"
patch:before="processor[#type='Sitecore.Pipelines.Loader.ShowVersion, Sitecore.Kernel']" />
</initialize>
</pipelines>
</sitecore>
</configuration>
On application restart, This is the output I got in the logs:
5764 14:38:08 INFO HttpModule is being initialized
5764 14:38:08 INFO
5764 14:38:08 INFO *********************************************************************
5764 14:38:08 INFO Debug StackTrace
5764 14:38:08 INFO at SCMVC72.Pipelines.DebugStackTrace.Process(PipelineArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Nexus.Web.HttpModule.Application_Start()
at Sitecore.Nexus.Web.HttpModule.Init(HttpApplication app)
at System.Web.HttpApplication.RegisterEventSubscriptionsWithIIS(IntPtr appContext, HttpContext context, MethodInfo[] handlers)
at System.Web.HttpApplication.InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context)
at System.Web.HttpApplicationFactory.GetSpecialApplicationInstance(IntPtr appContext, HttpContext context)
at System.Web.Hosting.PipelineRuntime.InitializeApplication(IntPtr appContext)
5764 14:38:08 INFO *********************************************************************
Which was output right befor the famous:
5764 14:38:08 INFO
5764 14:38:08 INFO **********************************************************************
5764 14:38:08 INFO **********************************************************************
5764 14:38:08 INFO Sitecore started
5764 14:38:08 INFO Sitecore.NET 7.2 (rev. 140228)
This confirms what HRH John West said when he wrote that "this happens in one of the obfuscated assemblies" - Sitecore.Nexus, to be more precise.

Answer 2

There is a SetActiveSite method in the Sitecore.Context class. This runs the Intialize pipeline, and there are a few places from which the method is called.
JobRunner.InitalizeContext
MediaCreator.SetContext
MediaRequestHandler.RedirectIfUserShouldBeLoggedIn
WebDavMediaRequestHandler.ProcessRequest
So it appears to be linked with initialization of a context, and therefore not just at the beginning of the application.
For information, I found this by searching for "Initalize" (with the quotes included), which found this:
CorePipeline.Run("initialize", new PipelineArgs());