Isapi was introduced with Microsofts Internet Server for generating dynamic web pages. It is the best choice for high performance web sites on a Windows platform (note: on a linux platform you'd use Apache shared modules), but this speed comes at a cost - it takes a considerable development effort to make even a simple change and any bugs in the Isapi DLL can easily crash your web server.

There are 2 types of Isapi technologies:
  • Isapi Filters process every incoming and/or outgoing request to the web server. Situations where this is appropriate are user authentication, altering the request headers and encrypted communication (i.e: SSL).
  • Isapi Extensions generate dynamic content. This is normally HTML, but often includes dynamic images and other custom formats. Two excellent examples of Isapi extensions are the asp.dll for Active Server Pages and the iscf.dll for Cold Fusion pages.