ISAPI Extension

An ISAPI extension is a dynamic link library (.dll) that uses ISAPI to provide a set of web functions above and beyond those natively provided by IIS. ISAPI is developed to provide advantage over the shortcomings of Common Gateway Interface, CGI. An ISAPI extension is a regular DLL file that exposes three special functions that are called by the calling process (i.e., IIS) and therefore, will be loaded to memory only once, irrespective of how many clients are going to use it at the same time.