HTTP Modules

HTTP Modules

HTTP handlers are endpoints for communication. Instances of handler classes consume HTTP requests and produce HTTP responses. HTTP modules are filters that process HTTP request and response messages as they pass through the pipeline, examining and possibly modifying their content. The pipeline uses HTTP modules to implement its own infrastructure, most notably security and session management.

HTTP modules are simply classes that implement the IHttpModule interface:

interface IHttpModule

{

  // called to attach module to app events

  void Init(HttpApplication app);

  // called to clean up

  void Dispose()

}

The Init method is called by an HttpApplication object when the module is first created. It gives the module the opportunity to attach one or more event handlers to the events exposed by the HttpApplication object.

The code in Figure 7 implements a simple HTTP module that handles its HttpApplication object's BeginRequest and EndRequest events and measures the elapsed time between them. In this example, the Init method uses normal .NET techniques to attach the module's OnBeginRequest and OnEndRequest handlers to the events fired by the HttpApplication object the module is being attached to. The implementation of OnBeginRequest simply stores the time when the event fired in the member variable starts. The implementation of OnEndRequest measures the elapsed time since OnBeginRequest was called and adds that information to the response message using a custom HTTP header. 

The OnEndRequest method takes advantage of the fact that the first parameter passed to an event handler is a reference to the object that fired the event; in this case, it is the HttpApplication object that the module is attached to. The HttpApplication object exposes the Http-Context object for the current message exchange as a property, which is exactly how OnEndRequest is able to manipulate the HTTP response message.

Once an HTTP module class is implemented, it must be deployed. Deployment involves two steps. As with an HTTP handler, you have to put the compiled module code either in the bin sub directory of your Web server's virtual directory or in the GAC so that the ASP.NET worker process can find it.

Then you have to tell the HTTP pipeline to create your module whenever a new HttpApplication object is created to handle a request sent to your application. You do this by adding an <httpModules> section to your virtual directory's Web.config file, as shown here:

<configuration>

 <system.web>

  <httpModules>

   <add

    name="Elapsed"

    type="Pipeline.ElapsedTimeModule, Pipeline"

   />

  </httpModules>

 </system.web>

</configuration>

In this example, the Web.config file tells the ASP.NET HTTP pipeline to attach an instance of the Pipeline.ElapsedTimeModule class to every HttpApplication object instantiated to service requests that target this virtual directory.