Difference between ISAPI filter and ASP .NET Http module

Question

Being as ASP .NET developer, today I was exploring internal working of IIS web server when this ISAPI filter hit me. I have an idea of ASP .NET web request inside ASP .NET run time where we have two concepts:

1. HTTP Modules : Acts as request filer before your request hits an HTTP handler. You can analyse http headers at this stage to take conditional decisions.
2. HTTP Handler : The end point of any http web request which does the actual processing of your *.aspx page request.

So I got confused between HTTP Modules and the ISAPI modules exposed by an Internet web server like IIS. Can someone help me understand the difference between the two? Also do they have any roots in the CGI web request serving paradigm from older era of web servers?

Answer 1

If you know what is the processing pipeline of IIS, then modules are registered at certain processing stages (authentication/authorization and so on) and work on all incoming requests.

However, filters are only used at a single stage (generate response) and usually applied to only a certain types of requests (unless wildcard).

HTTP modules are usually managed while ISAPI modules are unmanaged.