In this post, we would be going through HTTP request processing by IIS and components used by IIS for processing HTTP request. We would be referring to IIS 7 or later version for request processing.

Request processing for ASP.NET application starts with client’s browser sending HTTP or HTTPS request for a page or controller in case of ASP.NET MVC. When client’s browser request lands on Webserver, HTTP listener HTTP.sys (Hypertext Transfer Protocol Stack) receives request from network and transfer it IIS for actual processing of a request and returns output of request to client’s browser.

Post receiving request from network HTTP.sys transfers this request to WAS (Windows Process Activation Service). WAS is responsible for Managing Application Pool and Worker processes(w3wp.exe). WAS first request for information from Configuration Store. Configuration Store consist all Site, Directory and Access related information stored in ApplicationHost.config and Web.config files. WAS use Application Pool and site configuration information to configure HTTP.sys.

With HTTP.sys configuration done; WAS creates instance on Worker Process (w3wp.exe). WAS transfer HTTP request to Worker Process for Processing. In Worker Process, request passes through set of events such as Cache Resolution, State Management, Authentication and log writing (IIS logs). Worker Process map request with appropriate HTTPHandler, creates instance of handler and execute it.

If HTTP request requires to execute Managed Modules Worker Process creates AppDomain to facilitate requirement of Manage Modules.

Above, we discuss how IIS 7 is processing HTTP requests and providing response to client’s browser. We also put light on some of the components used in processing HTTP requests. Now I would like to fetch your attention towards Application Pool Mode. Though Application Pool and its configuration is part of Website hosting yet we would discuss implication of Application Pool Mode configuration on HTTP request processing.

So, what is Application Pool….

Application Pool is a process by which IIS separates or group different application/ websites. This is isolation level maintained by IIS. Different applications or websites which has same setting or configuration can be grouped under same Application Pool. There are two Managed Pipeline Modes in IIS, Integrated Application Pool and Classic Application Pool.

If we configure Application Pool under Classic Application Pool, request processing modules result in duplication of some processes such as Authentication and Authorization. If we configure Application Pool under Integrated Application Pool, request passes through ordered events which calls for only required events from native as well as manage modules. With Integrated Application Pool, we can remove duplicity of IIS and ASP.NET events in request processing.