[ACCEPTED]-What is ISAPI or ISAPI extension or filters? The more I read the more I am confused-asp.net

See e.g. here: http://searchwindowsserver.techtarget.com/definition/ISAPI

ISAPI (Internet Server Application 22 Program Interface) is a set of Windows program 21 calls that let you write a Web server 20 application that will run faster than 19 a common gateway interface (CGI) application. A 18 disadvantage of a CGI application (or 17 "executable file," as it is 16 sometimes called) is that each time it 15 is run, it runs as a separate process 14 with its own address space, resulting 13 in extra instructions that have to be 12 performed, especially if many instances 11 of it are running on behalf of users. Using 10 ISAPI, you create a dynamic link library 9 (DLL) application file that can run as 8 part of the Hypertext Transport Protocol (HTTP) application's 7 process and address space. The DLL files 6 are loaded into the computer when HTTP 5 is started and remain there as long as they 4 are needed; they don't have to be located 3 and read into storage as frequently as 2 a CGI application.

A special kind of ISAPI DLL is called an ISAPI filter, which can be designated to receive control for every HTTP request. You can create an ISAPI filter for encryption or decryption, for logging, for request screening, or for other purposes.

Or see another definition 1 with a graphical explanation here:

ISAPI Definition from PC Magazine

ISAPI! yup this thread is old but may be 22 my 2 cents worth something here.

ISAPI stands 21 for internet server application programmable 20 interface. As the name suggests this is 19 an interface provided in IIS for the developers. Where 18 you can tap into IIS core functionality 17 and either you can provide a custom functionality 16 in IIS using ISAPI extension (e.g. .net 15 dll) or ISPI filter (e.g. a custom file 14 uploader). There are set of built in ISAPI 13 api's for this.

Additionally building ISAPI 12 "extension" developments are a tough task 11 you need C++ and STL advance exposure. Mainly 10 allocate buffers for http post data and 9 need to be extremely careful about buffer 8 overflow errors and parsing posted data 7 as such error in ISAPI will bring down whole 6 IIS. Having said that once developed correctly 5 these extensions work pretty good. You can 4 also implement worker thread pool and custom 3 IIS load balancing etc.

But be prepared to 2 sleep under your working table I'm talking 1 by my own experience.

If you know that incoming HTTP messages 6 are handled by a pipeline (IIS/ASP.NET are 5 both part of the pipeline), you can treat 4 ISAPI/filters as components who extend this 3 pipeline.

As many ISAPI modules filter out 2 some messages, they are also called filters 1 naturally.

http://learn.iis.net/page.aspx/101/introduction-to-iis-7-architecture/

http://learn.iis.net/page.aspx/243/aspnet-integration-with-iis-7/

ISAPI filters are libraries loaded by the IIS web server. Every 6 incoming request and outgoing response passes 5 through the filters, and they're free to 4 perform any handling or translation they 3 wish. They can be used for authentication, content 2 transformation, logging, compression, and 1 myriads of other uses.

ISAPI is a framework/API that is provided 3 by Microsoft's web server, Internet Information 2 Services (IIS), that allows you to programmatically 1 inspect and alter web requests.