HTTP handler vs HTTP module

HttpHandler is where the request train is headed. HttpModule is a station along the way.

The two sentences:

An HttpModule will execute for every request to your application, regardless of extension, and is generally used for things like security, statistics, logging, etc.

An HttpHandler is generally associated with a specific extension, and is used for things like RSS feeds, dynamic image generation or modification, and the like.

A little more explanation if that's not completely clear:

The way I think about them - modules "plug in" to the request pipeline, whereas handlers "handle" a specific file extension. So, if you've got a site with a LoggingModule and a PdfHandler, both will execute for a request to http://example.com/sample.pdf, and the logging module alone will execute for a request to http://example.com/page.aspx.

There's a pretty clear article on the difference on MSDN: HTTP Handlers and HTTP Modules Overview

The prime and common goal of HttpHandler and HttpModule is to inject pre-processing logic before the ASP.NET request reaches the IIS Server.

ASP.NET provides two ways of injecting logic in the request pipeline;

1. Http Handlers: Http Handler helps us to inject pre-processing logic based on the extension of the file name requested. ASP.NET uses HTTP handlers for implementing a lot of its own functionality.For example, ASP.NET uses handlers for processing .aspx, .asmx and trace.axd files.

example: RSS feeds: To create an RSS feed for a Web site, you can create a handler that emits RSS-formatted XML. So when users send a request to your site that ends in .rss, ASP.NET calls your handler to process the request.

There are three steps involved in creating Handler 1. Implement IHttpHandler interface. 2. Register handler in web.config or machine.config file. 3. Map the file extension (*.arshad) to aspnet_isapi.dll in the IIS.

IHttpHandler interface has ProcessRequest method and IsReusable property which needs to be implemented. ProcessRequest: In this method, you write the code that produces the output for the handler. IsResuable: This property tells whether this handler can be reused or not.

You can register the handler in web.config file like this

<httpHandlers>
   <add verb="*" path="*.arshad" type="namespace.classname, assemblyname" />
</httpHandlers>

Note: here we are handling any file name with extension arshad.

2. Http Modules: HttpModule is an event based processor to inject pre-processing logic before the request reaches the IIS Server. ASP.NET uses HTTP Module to implement lots of its own functionality like authentication and authorization, session management and output caching etc.

ASP.NET engine emits lot of events as the request passess through the request pipeline. Some of those events are AuthenticateRequest, AuthorizeRequest, BeginRequest, EndRequest. By Using HttpModule you can write logic in these events. These logic get executed as the events fire and before the request reaches IIS.

There are two steps involved in creating Modules, 1. Implement IHttpModule interface 2. Register module in web.config or machine.config file

example: Security: Using HTTP module, you can perform custom authentication or other security checks before the request reaches IIS.