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7.1. What is ISAPI ISAPI is a multi-tier application programming interface for Microsoft Internet Information Services (IIS) web servers, and other compatible third-party web servers. Two application types exist for ISAPI applications: Extensions (full applications that run on IIS) Filters (applications that modify or enhance IIS functionality by constantly filtering for requests relevant to their functionality). You implement the ISAPI applications by compiling Extensions or Filters into Dynamic Link Library (DLL) files. The DLLs are registered with the web server which makes them available to use.