HttpHandlers and HttpModules
In ASP.old, you just couldn't do certain things using ASP. If you wanted to create something similar to the output caching in ASP.NET, you were forced to step outside ASP.old and use Internet Server API (ISAPI) filters. If you wanted to create a program that handled all files with a certain extension, you had to step outside ASP. If you wanted to write something that participated in the processing of each and every page, you had to step outside ASP.
One of the goals of ASP.NET was to allow you to do everything you could potentially conceive of related to Web programming directly in the product. It shouldn't limit you. To that end, Microsoft added two new concepts: HttpModules and HttpHandlers. These did not exist inside of ASP.old. To find analogous functionality, you had to step outside into the world of ISAPI programming. HttpModules and HttpHandlers are fairly similar to ISAPI filters, but they implement slightly different functionality.
An Overview of ASP.NET Request Handling
To understand how HttpModules and HttpHandlers fit into the scheme of things, you have to understand the way that ASP.NET handles a request. When a request is received by Internet Information Server (IIS), it looks at the extension to determine which ISAPI filter should handle the request. For any of the supported file extensions, such as .aspx or .asmx, the answer is aspnet_isapi.dll. When ASP.NET fires up, it performs almost the same process again. It looks at the request and compares it to the <httpHandlers> section of the .config file. By default, machine.config maps .aspx files to the PageHandlerFactory and .asmx files to the WebServiceHandlerFactory. This mapping determines the HttpHandler (class) that is responsible for handling the request. With the concept of mapping, you can create a new HttpHandler and map it to a new type of request. In fact, this is exactly what Microsoft did with Trace.axd. You will find that it is a new HttpHandler that is registered in machine.config for any request path that ends in trace.axd.
While processing a request received from IIS, ASP.NET raises several events. They are raised in the following order:
- Page Constructor
The items in bold represent several of the page-level events that are raised during the execution of a page. Each of these events can be sunk, providing opportunities to participate in the processing of each page in an application. In Chapter 7, "Security," we looked at handling the AuthenticateRequest and AuthorizeRequest events.