(ISAPI) microsoft's programming interface between applications and their internet server. Active Servers created with ISAPI extensions can be complete in-process applications themselves, or can "connect" to other services. ISAPI is used for the same sort of functions as cgi but uses microsoft windows dynamic link libraries (DLL) for greater efficiency. The server loads the DLL the first time a request is received and the DLL then stays in memory, ready to service other requests until the server decides it is no longer needed. This minimises the overhead associated with executing such applications many times.

An HTTP server can unload ISAPI application DLLs to free memory or preload them to speed up the first access. Applications can also be enhanced by isapi filters