Back to basics 7 : How Sharepoint works

A web application in SharePoint terminology is closely related to what is called a website in IIS terminology. An IIS website monitors for incoming requests through a particular port, and it checks for a particular host header or IP address, or both. Every SharePoint Foundation web application is hosted in an IIS website that has the same name as the web application. It can be helpful, especially when you are trying to see the relation between SharePoint and IIS from a high and broad perspective, to think of the SharePoint web application and its corresponding IIS website as a single entity. For one thing, although there is usually a one-to-one relation between SharePoint web applications and IIS websites, this is not always the case. It is possible to extend a SharePoint web application to multiple IIS websites, although that is not a common design. Just as in any application that is built on the ASP.NET-IIS integrated pipeline, when a front-end web server receives a request from a client for a page or other resource in a SharePoint site, the request is passed through a pipeline of units that process the request. This processing includes authenticating the user, verifying the user’s authorization, building the response, sending the response, and finally, logging the request.

The response to any request is produced by an HTTP handler object. Requests are assigned to one or another HTTP handler object (or to a handler factory class that creates HTTP handler objects) depending on the resource requested and the HTTP verb in the request. The assignment is determined by a stack of configuration files named applicationhost.config, machine.config, and web.config.

The request pipeline also contains HTTP modules. Modules are assemblies that typically contain one or more event handlers or define new events that other modules can handle. An HTTP module can register for one or more events in the life cycle of the request. They are often used to preprocess requests or postprocess the response. The result of a module’s preprocessing is stored in the HttpContext object. For example, the value of the User property is produced by an authentication module.

The processing of a request by HTTP modules and an HTTP handler is governed by an HttpApplication object or an object derived from that class. SharePoint installs a global.asax file in the root of each web application (IIS website) that identifies SPHttpApplication as the class from which an HTTP application object is created.