The Internet Server Application Programming Interface (ISAPI) is the API of Internet Information Services (IIS), Microsoft's collection of Windows-based web server services. The most prominent application of IIS and ISAPI is Microsoft's web server (like the more recent OS -- Windows Server 2003 which manages all server operations with efficiency). ISAPI was designed to model N-tier architecture.

The ISAPI has also been implemented by Apache's mod_isapi module so that server-side web applications written for Microsoft's IIS can be used with Apache.

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