Overview of Internet Information Services 5.0

ISAPI Extensions

ISAPI extensions are multithreaded DLLs that can be loaded into the same memory space (in-process) occupied by the Web service, and can perform server-side tasks as an interface between the user and IIS 5.0. ISAPI extensions have a strong performance advantage over CGI applications for several reasons:

ISAPI extensions can be configured so all of them run in a single process, or so they run in the same memory space as the Web service.
Instead of loading an executable for each request, ISAPI uses thread-safe DLLs that are loaded only once.
ISAPI uses Microsoft® Win32®-based APIs to communicate with the Web service, which are much faster than CGI methods.

IIS 5.0 does not have any built-in ISAPI extensions, so you either have to buy third-party extensions or customize your own. ISAPI extensions developed for other Web servers are generally easy to port to IIS 5.0. In addition, it is sometimes advantageous to rewrite existing CGI applications as ISAPI extensions to improve their performance. For more information, see Migrating a Web Server to IIS 5.0 in this book. For information about designing ISAPI extensions, see the “Designing High-Performance ISAPI Applications” topic in the IIS 5.0 online product documentation.