Beginner’s Guide: How IIS Process ASP.NET Request


When request come from client to the server a lot of operation is performed before sending response to the client. This is all about how IIS Process the request.  Here I am not going to describe the Page Life Cycle and there events, this article is all about the operation of IIS Level.  Before we start with the actual details, let’s start from the beginning so that each and everyone understand it’s details easily.  Please provide your valuable feedback and suggestion to improve this article.

What is Web Server ?

When we run our ASP.NET Web Application from visual studio IDE, VS Integrated ASP.NET Engine is responsible for executing all kind of requests and responses.  The process name is “WebDev.WebServer.Exe” which takes care of all request and response of a web application which is running from Visual Studio IDE.
Now, the name “Web Server” comes into picture when we want to host the application on a centralized location and wanted to access from many places. Web server is responsible for handle all the requests that are coming from clients, process them and provide the responses.

What is IIS ?

IIS (Internet Information Services) is one of the most powerful web servers from Microsoft that is used to host your ASP.NET Web application. IIS has its own ASP.NET Process Engine to handle the ASP.NET request. So, when a request comes from client to server, IIS takes that request and process it and send the response back to clients.

Request Processing :

Hope, till now it’s clear to you that what is the Web server and IIS is and what is the use of them. Now let’s have a look how they do things internally. Before we move ahead, you have to know about two main concepts

1.    Worker Process
2.   Application Pool

Worker Process:  Worker Process (w3wp.exe) runs the ASP.Net application in IIS. This process is responsible for managing all the request and response that are coming from the client system.  All the ASP.Net functionality runs under the scope of the worker process.  When a request comes to the server from a client worker process is responsible for generating the request and response. In a single word, we can say worker process is the heart of ASP.NET Web Application which runs on IIS.
Application Pool: Application pool is the container of the worker process.  Application pools are used to separate sets of IIS worker processes that share the same configuration.  Application pools enable a better security, reliability, and availability for any web application.  The worker process serves as the process boundary that separates each application pool so that when one worker process or application is having an issue or recycles, other applications or worker processes are not affected. This makes sure that a particular web application doesn’t impact other web application as they are configured into different application pools.

Application Pool with multiple worker processes is called “Web Garden.”
Now, I have covered all the basic stuff like the Web server, Application Pool, Worker process. Now let’s have a look how IIS process the request when a new request comes up from a client.
If we look into the IIS 6.0 Architecture, we can divide them into Two Layer

1.    Kernel Mode
2.    User Mode

Now, Kernel mode is introduced with IIS 6.0, which contains the HTTP.SYS.  So whenever a request comes from Client to Server, it will hit HTTP.SYS First.

Now, HTTP.SYS is Responsible for pass the request to the particular Application pool. Now here is one question, How HTTP.SYS does come to know where to send the request?  This is not a random pickup. Whenever we create a new Application Pool, the ID of the Application Pool is being generated, and it’s registered with the HTTP.SYS. So whenever HTTP.SYS Received the request from any web application, it checked for the Application Pool and based on the application pool it sends the request.

So, this was the first steps of IIS Request Processing.
Till now, Client Requested for some information and request came to the Kernel level of IIS means at HTTP.SYS. HTTP.SYS has been identified the name of the application pool where to send. Now, let’s see how this request moves from HTTP.SYS to Application Pool.

In User Level of IIS, we have Web Admin Services (WAS) which takes the request from HTTP.SYS and pass it to the respective application pool.

When Application pool receives the request, it just passes the request to worker process (w3wp.exe). The worker process “w3wp.exe” looks up the URL of the request to load the correct ISAPI extension. ISAPI extensions are the IIS way to handle requests for different resources. Once ASP.NET is installed, it installs its own ISAPI extension (aspnet_isapi.dll) and adds the mapping into IIS.
Note: Sometimes if we install IIS after installing, we need to register the extension with IIS using an aspnet_regiis command.

When Worker process loads the aspnet_isapi.dll, it starts an HTTPRuntime, which is the entry point of an application. HTTPRuntime is a class which calls the ProcessRequest method to start Processing.

When this method called, a new instance of HTTPContext is created.  Which is accessible using HTTPContext.Current  Properties. This object remains alive during the life time of object request.  Using HttpContext.Current we can access some other objects like Request, Response, Session, etc.

After that HttpRuntime load, an HttpApplication object with the help of  HttpApplicationFactory class. Every request should pass through the corresponding HTTPModule to reach to HTTPHandler, this list of a module is configured by the HTTPApplication.
Now, the concept comes called “HTTPPipeline.” It is called a pipeline because it contains a set of HttpModules ( For Both Web.config and Machine.config level) that intercept the request on its way to the HttpHandler. HTTPModules are classes that have access to the incoming request. We can also create our HTTPModule if we need to handle anything during upcoming request and response.

HTTP Handlers are the endpoints in the HTTP pipeline. All request that is passing through the HTTPModule should reach to HTTPHandler.  The  HTTP Handler generates the output for the requested resource. So, when we were requesting for any aspx web pages,   it returns the corresponding HTML output.
All the request now passes from httpModule to respective HTTPHandler then the method and the ASP.NET Page life cycle starts.  This ends the IIS Request processing and starts the ASP.NET Page Lifecycle.


When the client request for some information from a web server, request first reaches to HTTP.SYS of IIS. HTTP.SYS then send the request to particular  Application Pool. Application Pool then forwards the request to worker process to load the ISAPI Extension which will create an HTTPRuntime Object to Process the request via HTTPModule and HTTP handler. After that, the ASP.NET Page LifeCycle events start.
This was a just overview of IIS Request Processing to let Beginner’s know how the request gets processed in the backend.  If you want to learn about details, please check the link for Reference and Further Study section.

Reference and Further Study

A low-level Look at the ASP.NET Architecture
IIS Architecture

Please share your suggestion and feedback.

ASP.NET, General, IIS , ,


  1. Dear Abhijit,

    Thank you for the great article. I must accept that I learnt something new about IIS request processing from this article that I didn’t know earlier.

    Awesome article!

    Keep it up and take care.

    Sheo Narayan


  2. Hi Abhijit,

    Really this is very good articles. It helps a lot for beginers.
    Appreciate for your efforts and sharing the knowledge to dotnet community.

    I know personally, how much hard to prepare the documents.

    Ramesh Babu


  3. Thanks Abhijeet,

    It is very nice article. It is really helpful for understanding the IIS level request processing.

    Pravin Shimpi


  4. Nice article

    Abhijit I would like to see video for the same article.

    if it is possible please reply me on my emailid


  5. Hi Abhijit,

    This was really very Good. I was really afraid about IIS because it was something which I didn’t understand ever but this time I got a good way of understanding it.

    Thanks for writing this Awesome article.



  6. Hi Abhijit,

    This was really very Good. I was really afraid about IIS because it was something which I didn’t understand ever but this time I got a good way of understanding it.

    Thanks for writing this Awesome article.



  7. Awesome article……Its so informative and very good for beginners to learn without any confusion…..just follow it step by step……….I ll suggest it to my friends………..Thanks for the article one again…..


  8. Really a nice artical for all, excellent learning for beginners and refreshment for experts 🙂

    Thanks Abhijit.


  9. This is an ultimate explanation one can give on life cycle …..ULTIMATE..superb explanation..Hats off to U..:) ..I am lucky dat i got to read this article..Ever after reading soo many other articles i still had one or the other doubt running in my mind..But after reading yo article..Thanks to U..u cleared all my doubts..I am very very Happy today for i was longing for this kinda explanation..Thank U soo much once again..Keep writing such awesome articles..:)WISH U ALL GOOD LUCK..




  11. Hi Abhijit,
    This is really a good article of HTTP request pipeline. But i have one question… WAS is really stand on Web Admin Services(WAS) or Windows Process Activation Service (WAS)? Can you clear me.


  12. Thanks Abhijeet,

    It is very nice article. It is really helpful for understanding the IIS level request processing.


    Arindam M


  13. HI Abhijit,
    Its really a wonderful article. Thanks a lot.
    It wud be really great, if you can give me more clear understanding on Application Pool and Worker process. Actually my biggest confusion in .net is on the working of Worker process. and the architecture of application pool aaplication domain, worker process and threads 😦 can you explain it a bit, it wud be reallt helpful.

    Thanks a lot


  14. Thanks a lot for this extremely informative and interesting article, Abhijit. You have certainly explained the concept in a very simple way which is easily understood.


  15. Excellent……Just started ASP.Net and going through an article like this has already reduced lot of confusion……Thank u so much…..This blog is really precious for beginner……


  16. bur sir..when a user processes that page and again send the page for processing to the server… does the the IIS recognises that particular user?


  17. Pingback: Guru N Guns's
    1. Thanks Ajay. Could you please let me know what do you mean by “structure of worker process under application pool” / or please rephrase your question ? It will help me to answer .


  18. This is best article man..It gives Crux of ASP.NET..
    I have been following your articles,they are easy to understand and very useful..
    Thanks Bro..


  19. You really make it appear really easy together with your presentation but I find this topic to be actually something which I believe I would by no means understand.
    It sort of feels too complex and extremely extensive for
    me. I’m looking forward for your subsequent publish, I
    will attempt to get the dangle of it!


  20. appreciated for your attempt really wonderful article which helped me a lot,so many doubts are cleared…Thanks


  21. Quite an interesting article, and well done for sharing. I am trying to get a feel for how the IIS architecture works when deploying to a webhost. Any pointers?


  22. Really Nice article but i was looking for concept of creating multiple Worker process in Application pool as we have depicted in picture.Although i found and learned it.Inclusion of that point would make this article more details.No questions would be left.Thanks for such a detailed basic subject.


  23. Dear Abhijit,

    Excellent article. It’s very simple and clear view. Thank you so much for your efforts on this.

    Behin Vincent


  24. Could you please also include some explanation about how a request from and mvc application is identified and handled by IIS ? That would really help.

    Nice Article. Thanks !!


  25. It is very easy to understand.. Thanks Abhijit. Is there any article to understand how Sessions are being handled in IIS Level?


  26. Very nice article, and clear understanding. Its just not for beginners, but also for experienced developers to quickly brush up the fundamentals. Thanks..
    On a side note, just wondering how some comments in this article are of year Nov-1999?


  27. What an explanation!!!
    Simple and clear language used for explaining this vast topic. It took hardly 15 minutes to understand this concept. I am sure one can’t find any better article than this on “How IIS Process ASP.NET Request”.

    Thanks a lot. 🙂


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