A SharePoint Consultant's Blog : Think Parallel

Whenever we create a new WSS web application in SharePoint, behind the scene a new IIS Web site gets created. Now to make it SharePoint specific, WSS does the following 

• WSS makes few entries into the IIS Metabase. (IIS Metabase stores conifiguration information about it IIS Web Sites and Virtual Directories)
• Creates virtual directories which maps to 12 hive (installation directory\Program Files\Common Files\Microsoft Shared\Web Server Extensions\12\)
  • _controltemplates – where user controls are kept.
  • _layouts – has application pages in it.
  • _vti_bin – dlls and web services repository
  • _wpresources – resource file repository for Web Parts
• Adds WSS-specific web.config to the root directory of the IIS Web site.
• Adds wildcard application map to route all incoming to aspnet_isapi.dll. (Right click your iis web site, select home directory, click on Configuration – there we could find the wildcard application map entry.

Finally WSS extends the routing architecture of IIS and ASP.NET to properly route all incoming requests through WSS runtime.

IIS maps an incoming request to a physical file within root directory of an IIS Web Site or virtual directory. (Virtual directory – an entity defining a child URL space nested within the URL space of its parent IIS Web Site. IIS provides the flexibility of defining the root directory for a virtual directory at any location)

IIS supports Internet Server Application Programming Interface (ISAPI) programming model, it allows us to configure an IIS Web Site or virtual directory so that incoming requests trigger the execution of the custom code on the Web Server.

ISAPI programming models consists of ISAPI extension and ISAPI filters.

ISAPI extension is a dll that serves as an endpoint for all the incoming requests.

More on ISAPI extension

http://msdn.microsoft.com/en-us/library/ms525172.aspx

ISAPI filter-acts as an interceptor. It provides pre as well post processing for each and every incoming request. These extensions are however difficult to develop as they are written in C++.

More on ISAPI filters

http://msdn.microsoft.com/en-us/library/ms524610.aspx

For better understanding of low level look at ASP.NET architecture

http://www.west-wind.com/presentations/howaspnetworks/howaspnetworks.asp

In short, ASP.NET framework is implemented as an ISAPI extension named aspnet_isapi.dll. Whenever IIS sees an incoming request targeting a file with extension like .aspx,.ascx,.asmx, based on the application map defined, it forwards the request to aspnet_isapi.dll, which effectively passes control over to ASP.NET framework.

ASP.NET framework compiles an .aspx page to .dll. It parses the .aspx file to generate a C# file that inherits from Page class. Once the ASP.NET page parser builds the source c# file for an .aspx page, it than compile it into a dll. This occurs only the first time when the page is requested; afterwards the same dll is used for all the subsequent requests that target the same page.

Now comes into picture HTTP request pipeline exposed by ASP.NET framework. It provides the developer with a degree of control comparable with ISAPI programming model. Http Request Pipeline contains HttpHandler, HttpApplication nad HttpModule components.

Once a request comes into the AppDomain managed by the ASP.NET runtime, ASP.NET uses the HttpWorkerRequest class to store the request information. Following that, the runtime wraps the request’s information in a class named HttpContext. The HttpContext class includes all the information you’d ever want to know about a request, including references to the current request’s HttpRequest and HttpResponse objects. The runtime produces an instance of HttpApplication (if one is not already available) and then fires a number of application-wide events (such as BeginRequest and AuthenticateRequest). These events are also pumped through any HttpModules attached to the pipeline. Finally, ASP.NET figures out what kind of handler is required to handle the request, creates one, and asks the handler to process the request. After the handler deals with the request, ASP.NET fires a number of post-processing events (like EndRequest) through the HttpApplication object and the HttpModules.

HttpApplication – During the lifetime of a Web application, the HttpApplication objects serve as places to hold application-wide data and handle application-side events.

HttpModules – While the Application object is suitable for handling application-wide events and data on a small scale, sometimes application-wide tasks need a little heavier machinery. HttpModules serve that purpose.ASP.NET includes a number of predefined HttpModules. For example, session state, authentication, and authorization are handled via HttpModules.

HttpHandlers -The last stop a request makes in the pipeline is an HttpHandler. Any class implementing the interface IHttpHandler qualifies as a handler. When a request finally reaches the end of the pipeline, ASP.NET consults the configuration file to see if the particular file extension is mapped to an HttpHandler. If it is, the ASP.NET loads the handler and calls the handler’s IHttpHandler.ProcessRequest method to execute the request.

ASP.NET includes several HTTPHandlers already, including System.Web.UI.Page and System.Web.Services.WebService

http://www.brainbell.com/tutorials/ASP/The_ASP.NET_Pipeline.html

WSS uses the above ASP.NET technique to extend the HTTP Request Pipeline.

Configures each web application with custom HttpApplication object using SPHttpApplication class. This class is within Microsoft.SharePoint.dll. It creates a custom global.asax file at the root of Web Application that inherits from SPHttpApplication.

<%@ Assembly Name=”Microsoft.SharePoint”%><%@ Application Language=”C#” Inherits=”Microsoft.SharePoint.ApplicationRuntime.SPHttpApplication” %>

It also makes use of custom HttpHandler and HttpModule as well.

We could find their entries in web.config of the web applciation

  <httpHandlers>

      <remove verb=“GET,HEAD,POST“ path=“*“ />

      <add verb=“GET,HEAD,POST“ path=“*“ type=“Microsoft.SharePoint.ApplicationRuntime.SPHttpHandler, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c“ />

      <add verb=“OPTIONS,PROPFIND,PUT,LOCK,UNLOCK,MOVE,COPY,GETLIB,PROPPATCH,MKCOL,DELETE,(GETSOURCE),(HEADSOURCE),(POSTSOURCE)“ path=“*“ type=“Microsoft.SharePoint.ApplicationRuntime.SPHttpHandler, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c“ />

// other handlers

    </httpHandlers>

<httpModules>

      <clear />

      <add name=“SPRequest“ type=“Microsoft.SharePoint.ApplicationRuntime.SPRequestModule, Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c“ />

// other http modules

    </httpModules>

And finally it makes use of SPVirtualPathProvider, which abstracts the details of where page files are stored away from the ASP.NET runtime, using which the pages are served from content database. Once the content are retrieved by Virtual path provider it is passed to asp.net runtime for parsing. SPRequestModule component contains code to register SPVirutalPathProvider class with ASP.NET framework. SPVirutalPathProvider class works toghether with SPPageParserFilter to supply processing instructions to ASP.NET page parser whether to compile the aspx page to dll or process it in no compile mode.

More on virual path provider

http://weblogs.asp.net/scottgu/archive/2005/11/27/431650.aspx

Twice in the past week I have had the issue in my MOSS code where it is throwing an exception: “The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again”

I recently had an issue where I attached a SharePoint event handler to a list item, and when the list item was approved it fired off a group email. Problem was that if the user who approved the item did not have super powered privileges on the system, the event handler would fail due to a permissions error. 

To run the SPSecurity.RunWithElevatedPrivileges method