Introduction
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 to execute all kind of asp.net requests and responses. The process name is "WebDev.WebServer.Exe" which actually takw care of all request and response of an web application which is running from Visual Studio IDE.Now, the name “Web Server” come into picture when we want to host the application on a centralized location and wanted to access from many locations. 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 Server) is one of the most powerful web servers from Microsoft that is used to host your ASP.NET Web application. IIS has it's 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 response back to clients. 
Request Processing :
Hope, till now it’s clear to you that what is 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 to manage all the request and response that are coming from client system. All the ASP.Net functionality runs under the scope of worker process. When a request comes to the server from a client worker process is responsible to generate 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 worker process. Application pools is used to separate sets of IIS worker processes that share the same configuration. Application pools enables 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 not impact other web application as they they are configured into different application pools.
Application Pool with multiple worker process is called “Web Garden”.
Now, I have covered all the basic stuff like Web server, Application Pool, Worker process. Now let’s have look how IIS process the request when a new request comes up from client.
If we look into the IIS 6.0 Architecture, we can divided them into Two Layer
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.
1. Kernel Mode
2. User Mode
Now, HTTP.SYS is Responsible for pass the request to particular Application pool. Now here is one question, How HTTP.SYS comes to know where to send the request? This is not a random pickup. Whenever we creates 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 checks for the Application Pool and based on the application pool it send 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 receive the request, it simply pass the request to worker process (w3wp.exe) . The worker process “w3wp.exe” looks up the URL of the request in order 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 asp.net, we need to register the extension with IIS using aspnet_regiis command.
When Worker process loads the aspnet_isapi.dll, it start an HTTPRuntime, which is the entry point of an application. HTTPRuntime is a class which calls the ProcessRequest method to start Processing.
When this methods called, a new instance of HTTPContext is been created. Which is accessible using HTTPContext.Current Properties. This object still remains alive during life time of object request. Using HttpContext.Current we can access some other objects like Request, Response, Session etc.
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 own HTTPModule if we need to handle anything during upcoming request and response.
All the request now passes from httpModule to respective HTTPHandler then method and the ASP.NET Page life cycle starts. This ends the IIS Request processing and start the ASP.NET Page Lifecycle.
Conclusion
When client request for some information from a web server, request first reaches to HTTP.SYS of IIS. HTTP.SYS then send the request to respective Application Pool. Application Pool then forward the request to worker process to load the ISAPI Extension which will create an HTTPRuntime Object to Process the request via HTTPModule and HTTPHanlder. After that the ASP.NET Page LifeCycle events starts.This was just overview of IIS Request Processing to let Beginner’s know how the request get processed in backend. If you want to learn in details please check the link for Reference and further Study section.
Reference and Further Study
A low-level Look at the ASP.NET ArchitectureIIS Architecture
by Abhijit Jana
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