A low-level Look at the ASP.NET Architecture - Page 2

By Rick Strahl
West Wind Technologies
rstrahl@west-wind.com

Getting into the .NET runtime

The actual entry points into the .NET Runtime occur through a number of undocumented classes and interfaces. Little is known about these interfaces outside of Microsoft, and Microsoft folks are not eager to talk about the details, as they deem this an implementation detail that has little effect on developers building applications with ASP.NET.

The worker processes ASPNET_WP.EXE (IIS5) and W3WP.EXE (IIS6) host the .NET runtime and the ISAPI DLL calls into small set of unmanged interfaces via low level COM that eventually forward calls to an instance subclass of the ISAPIRuntime class. The first entry point to the runtime is the undocumented ISAPIRuntime class which exposes the IISAPIRuntime interface via COM to a caller. These COM interfaces low level IUnknown based interfaces that are meant for internal calls from the ISAPI extension into ASP.NET. Figure 3 shows the interface and call signatures for the IISAPIRuntime interface as shown in Lutz Roeder's excellent .NET Reflector tool (http://www.aisto.com/roeder/dotnet/). Reflector an assembly viewer and disassembler that makes it very easy to look at medadata and disassembled code (in IL, C#, VB) as shown in Figure 3. It's a great way to explore the bootstrapping process.



The IISAPIRuntime interface acts as the interface point between the unmanaged code coming from the ISAPI extension (directly in IIS 6 and indirectly via the Named Pipe handler in IIS 5). If you take a look at this class you'll find a ProcessRequest method with a signature like this:

return: MarshalAs(UnmanagedType.I4)]
int ProcessRequest([In] IntPtr ecb,
                   [In, MarshalAs(UnmanagedType.I4)] int useProcessModel);

The ecb parameter is the ISAPI Extension Control Block (ECB) which is passed as an unmanaged resource to ProcessRequest. The method then takes the ECB and uses it as the base input and output interface used with the Request and Response objects. An ISAPI ECB contains all low level request information including server variables, an input stream for form variables as well as an output stream that is used to write data back to the client. The single ecb reference basically provides access to all of the functionality an ISAPI request has access to and ProcessRequest is the entry and exit point where this resource initially makes contact with managed code.

The ISAPI extension runs requests asynchronously. In this mode the ISAPI extension immediately returns on the calling worker process or IIS thread, but keeps the ECB for the current request alive. The ECB then includes a mechanism for letting ISAPI know when the request is complete (via ecb.ServerSupportFunction) which then releases the ECB. This asynchronous processing releases the ISAPI worker thread immediately, and offloads processing to a separate thread that is managed by ASP.NET.

ASP.NET receives this ecb reference and uses it internally to retrieve information about the current request such as server variables, POST data as well as returning output back to the server. The ecb stays alive until the request finishes or times out in IIS and ASP.NET continues to communicate with it until the request is done. Output is written into the ISAPI output stream (ecb.WriteClient()) and when the request is done, the ISAPI extension is notified of request completion to let it know that the ECB can be freed. This implementation is very efficient as the .NET classes essentially act as a fairly thin wrapper around the high performance, unmanaged ISAPI ECB.

Loading .NET - somewhat of a mystery

Let's back up one step here: I skipped over how the .NET runtime gets loaded. Here's where things get a bit fuzzy. I haven't found any documentation on this process and since we're talking about native code there's no easy way to disassemble the ISAPI DLL and figure it out.

My best guess is that the worker process bootstraps the .NET runtime from within the ISAPI extension on the first hit against an ASP.NET mapped extension. Once the runtime exists, the unmanaged code can request an instance of an ISAPIRuntime object for a given virtual path if one doesn't exist yet. Each virtual directory gets its own AppDomain and within that AppDomain the ISAPIRuntime exists from which the bootstrapping process for an individual application starts. Instantiation appears to occur over COM as the interface methods are exposed as COM callable methods.

To create the ISAPIRuntime instance the System.Web.Hosting.AppDomainFactory.Create() method is called when the first request for a specific virtual directory is requested. This starts the 'Application' bootstrapping process. The call receives parameters for type and module name and virtual path information for the application which is used by ASP.NET to create an AppDomain and launch the ASP.NET application for the given virtual directory. This HttpRuntime derived object is created in a new AppDomain. Each virtual directory or ASP.NET application is hosted in a separate AppDomain and they get loaded only as requests hit the particular ASP.NET Application. The ISAPI extension manages these instances of the HttpRuntime objects, and routes inbound requests to the right one based on the virtual path of the request.



Back in the runtime At this point we have an instance of ISAPIRuntime active and callable from the ISAPI extension. Once the runtime is up and running the ISAPI code calls into the ISAPIRuntime.ProcessRequest() method which is the real entry point into the ASP.NET Pipeline. The flow from there is shown in Figure 4.

Remember ISAPI is multi-threaded so requests will come in on multiple threads through the reference that was returned by ApplicationDomainFactory.Create(). Listing 1 shows the disassembled code from the IsapiRuntime.ProcessRequest method that receives an ISAPI ecb object and server type as parameters. The method is thread safe, so multiple ISAPI threads can safely call this single returned object instance simultaneously.

Listing 1: The Process request method receives an ISAPI Ecb and passes it on to the Worker request

public int ProcessRequest(IntPtr ecb, int iWRType)
{
      HttpWorkerRequest request1 = ISAPIWorkerRequest.CreateWorkerRequest(ecb, iWRType);
      string text1 = request1.GetAppPathTranslated();
      string text2 = HttpRuntime.AppDomainAppPathInternal;
      if (((text2 == null) || text1.Equals(".")) ||
         (string.Compare(text1, text2, true, CultureInfo.InvariantCulture) == 0))
      {
            HttpRuntime.ProcessRequest(request1);
            return 0;
      }
      HttpRuntime.ShutdownAppDomain("Physical application path changed from " +
            text2 + " to " + text1);
      return 1;
}

The actual code here is not important, and keep in mind that this is disassembled internal framework code that you'll never deal with directly and that might change in the future. It's meant to demonstrate what's happening behind the scenes. ProcessRequest receives the unmanaged ECB reference and passes it on to the ISAPIWorkerRequest object which is in charge of creating the Request Context for the current request as shown in Listing 2.

The System.Web.Hosting.ISAPIWorkerRequest class is an abstract subclass of HttpWorkerRequest, whose job it is to create an abstracted view of the input and output that serves as the input for the Web application. Notice another factory method here: CreateWorkerRequest, which as a second parameter receives the type of worker request object to create. There are three different versions: ISAPIWorkerRequestInProc, ISAPIWorkerRequestInProcForIIS6, ISAPIWorkerRequestOutOfProc. This object is created on each incoming hit and serves as the basis for the Request and Response objects which will receive their data and streams from the data provided by the WorkerRequest.

The abstract HttpWorkerRequest class is meant to provide a highlevel abstraction around the low level interfaces so that regardless of where the data comes from, whether it's a CGI Web Server, the Web Browser Control or some custom mechanism you use to feed the data to the HTTP Runtime. The key is that ASP.NET can retrieve the information consistently.

In the case of IIS the abstraction is centered around an ISAPI ECB block. In our request processing, ISAPIWorkerRequest hangs on to the ISAPI ECB and retrieves data from it as needed. Listing 2 shows how the query string value is retrieved for example.

Listing 2: An ISAPIWorkerRequest method that uses the unmanged

// *** Implemented in ISAPIWorkerRequest
public override byte[] GetQueryStringRawBytes()
{
      byte[] buffer1 = new byte[this._queryStringLength];
      if (this._queryStringLength > 0)
      {
            int num1 = this.GetQueryStringRawBytesCore(buffer1, this._queryStringLength);
            if (num1 != 1)
            {
                  throw new HttpException( "Cannot_get_query_string_bytes");
            }
      }
      return buffer1;
}
// *** Implemented in a specific implementation class ISAPIWorkerRequestInProcIIS6
internal override int GetQueryStringCore(int encode, StringBuilder buffer, int size)
{
      if (this._ecb == IntPtr.Zero)
      {
            return 0;
      }
      return UnsafeNativeMethods.EcbGetQueryString(this._ecb, encode, buffer, size);
}

ISAPIWorkerRequest implements a high level wrapper method, that calls into lower level Core methods, which are responsible for performing the actual access to the unmanaged APIs - or the 'service level implementation'. The Core methods are implemented in the specific ISAPIWorkerRequest instance subclasses and thus provide the specific implementation for the environment that it's hosted in. This makes for an easily pluggable environment where additional implementation classes can be provided later as newer Web Server interfaces or other platforms are targeted by ASP.NET. There's also a helper class System.Web.UnsafeNativeMethods. Many of these methods operate on the ISAPI ECB structure performing unmanaged calls into the ISAPI extension.

HttpRuntime, HttpContext, and HttpApplication - Oh my

When a request hits, it is routed to the ISAPIRuntime.ProcessRequest() method. This method in turn calls HttpRuntime.ProcessRequest that does several important things (look at System.Web.HttpRuntime.ProcessRequestInternal with Reflector):

• Create a new HttpContext instance for the request
• Retrieves an HttpApplication Instance
• Calls HttpApplication.Init() to set up Pipeline Events
• Init() fires HttpApplication.ResumeProcessing() which starts the ASP.NET pipeline processing

First a new HttpContext object is created and it is passed the ISAPIWorkerRequest that wrappers the ISAPI ECB. The Context is available throughout the lifetime of the request and ALWAYS accessible via the static HttpContext.Current property. As the name implies, the HttpContext object represents the context of the currently active request as it contains references to all of the vital objects you typically access during the request lifetime: Request, Response, Application, Server, Cache. At any time during request processing HttpContext.Current gives you access to all of these object.

The HttpContext object also contains a very useful Items collection that you can use to store data that is request specific. The context object gets created at the begging of the request cycle and released when the request finishes, so data stored there in the Items collection is specific only to the current request. A good example use is a request logging mechanism where you want to track start and end times of a request by hooking the Application_BeginRequest and Application_EndRequest methods in Global.asax as shown in Listing 3. HttpContext is your friend - you'll use it liberally if you need data in different parts of the request or page processing.

Listing 3 - Using the HttpContext.Items collection lets you save data between pipeline events

protected void Application_BeginRequest(Object sender, EventArgs e)
{
      //*** Request Logging
      if (App.Configuration.LogWebRequests)
            Context.Items.Add("WebLog_StartTime",DateTime.Now);
}
protected void Application_EndRequest(Object sender, EventArgs e)
{
      // *** Request Logging
      if (App.Configuration.LogWebRequests)
      {
            try
            {    
                  TimeSpan Span = DateTime.Now.Subtract(
                                 (DateTime) Context.Items["WebLog_StartTime"] );
                  int MiliSecs = Span.TotalMilliseconds;
                  // do your logging
                  WebRequestLog.Log(App.Configuration.ConnectionString,
                                    true,MilliSecs);
      }
}

Once the Context has been set up, ASP.NET needs to route your incoming request to the appropriate application/virtual directory by way of an HttpApplication object. Every ASP.NET application must be set up as a Virtual (or Web Root) directory and each of these 'applications' are handled independently.



Master of your domain: HttpApplication

Each request is routed to an HttpApplication object. The HttpApplicationFactory class creates a pool of HttpApplication objects for your ASP.NET application depending on the load on the application and hands out references for each incoming request. The size of the pool is limited to the setting of the MaxWorkerThreads setting in machine.config's ProcessModel Key, which by default is 20.

The pool starts out with a smaller number though; usually one and it then grows as multiple simulataneous requests need to be processed. The Pool is monitored so under load it may grow to its max number of instances, which is later scaled back to a smaller number as the load drops.

HttpApplication is the outer container for your specific Web application and it maps to the class that is defined in Global.asax. It's the first entry point into the HTTP Runtime that you actually see on a regular basis in your applications. If you look in Global.asax (or the code behind class) you'll find that this class derives directly from HttpApplication:

public class Global : System.Web.HttpApplication

HttpApplication's primary purpose is to act as the event controller of the Http Pipeline and so its interface consists primarily of events. The event hooks are extensive and include:

• BeginRequest
• AuthenticateRequest
• AuthorizeRequest
• ResolveRequestCache
• AquireRequestState
• PreRequestHandlerExecute
• :Handler Execution:
• PostRequestHandlerExecute
• ReleaseRequestState
• UpdateRequestCache
• EndRequest

Each of these events are also implemented in the Global.asax file via empty methods that start with an Application_ prefix. For example, Application_BeginRequest(), Application_AuthorizeRequest(). These handlers are provided for convenience since they are frequently used in applications and make it so that you don't have to explicitly create the event handler delegates.

It's important to understand that each ASP.NET virtual application runs in its own AppDomain and that there inside of the AppDomain multiple HttpApplication instances running simultaneously, fed out of a pool that ASP.NET manages. This is so that multiple requests can process at the same time without interfering with each other.

To see the relationship between the AppDomain, Threads and the HttpApplication check out the code in Listing 4.

Listing 4 - Showing the relation between AppDomain, Threads and HttpApplication instances

private void Page_Load(object sender, System.EventArgs e)
{
      // Put user code to initialize the page here
      this.ApplicationId = ((HowAspNetWorks.Global)
                  HttpContext.Current.ApplicationInstance).ApplicationId ;
      this.ThreadId = AppDomain.GetCurrentThreadId();
      this.DomainId = AppDomain.CurrentDomain.FriendlyName;
      this.ThreadInfo = "ThreadPool Thread: " +
            System.Threading.Thread.CurrentThread.IsThreadPoolThread.ToString() +
                            "<br>Thread Apartment: " +
            System.Threading.Thread.CurrentThread.ApartmentState.ToString();
      // *** Simulate a slow request so we can see multiple
      //     requests side by side.
      System.Threading.Thread.Sleep(3000);
}

This is part of a demo is provided with your samples and the running form is shown in Figure 5. To check this out run two instances of a browser and hit this sample page and watch the various Ids.



You'll notice that the AppDomain ID stays steady while thread and HttpApplication Ids change on most requests, although they likely will repeat. HttpApplications are running out of a collection and are reused for subsequent requests so the ids repeat at times. Note though that Application instance are not tied to a specific thread - rather they are assigned to the active executing thread of the current request.

Threads are served from the .NET ThreadPool and by default are Multithreaded Apartment (MTA) style threads. You can override this apartment state in ASP.NET pages with the ASPCOMPAT="true" attribute in the @Page directive. ASPCOMPAT is meant to provide COM components a safe environment to run in and ASPCOMPAT uses special Single Threaded Apartment (STA) threads to service those requests. STA threads are set aside and pooled separately as they require special handling.

The fact that these HttpApplication objects are all running in the same AppDomain is very important. This is how ASP.NET can guarantee that changes to web.config or individual ASP.NET pages get recognized throughout the AppDomain. Making a change to a value in web.config causes the AppDomain to be shut down and restarted. This makes sure that all instances of HttpApplication see the changes made because when the AppDomain reloads the changes from ASP.NET are re-read at startup. Any static references are also reloaded when the AppDomain so if the application reads values from App Configuration settings these values also get refreshed.

To see this in the sample, hit the ApplicationPoolsAndThreads.aspx page and note the AppDomain Id. Then go in and make a change in web.config (add a space and save). Then reload the page. You'll l find that a new AppDomain has been created.

In essence the Web Application/Virtual completely 'restarts' when this happens. Any requests that are already in the pipeline processing will continue running through the existing pipeline, while any new requests coming in are routed to the new AppDomain. In order to deal with 'hung requests' ASP.NET forcefully shuts down the AppDomain after the request timeout period is up even if requests are still pending. So it's actually possible that two AppDomains exist for the same HttpApplication at a given point in time as the old one's shutting down and the new one is ramping up. Both AppDomains continue to serve their clients until the old one has run out its pending requests and shuts down leaving just the new AppDomain running.

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