I have this requirement to write a customized module in http server. Newer version of the application uses the APACHE http server and the older version is deployed on IHS (IBM Http Server). I tried googling it but I couldn't find anywhere if I can write a module and packaged it in IHS .Is that possible ? If yes , how ? Do I have to get the code and build it with my module or I can write some extension and plugin into IHS. anything will help , thanks.
Yes, you extend IHS the same way you extend a vanilla Apache HTTP Server module -- with the Apache HTTP Server API.
You can either use binary modules compiled against vanilla Apache, or compile directly against IHS with the inclued bin/apxs script.
For pointers, look at any existing module in the core distribution of Apache, or Nick Kew's Apache Modules book.