Internet Server API (ISAPI)

Definition of Internet Server API (ISAPI) in The Network Encyclopedia.

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What is ISAPI (Internet Server API)?

A Microsoft set of standard application programming interfaces (APIs) for developing extensions to Internet Information Services (IIS). Internet Server API (ISAPI) provides developers with a powerful way to extend the functionality of IIS to provide better performance than Active Server Pages (ASP) or the Common Gateway Interface (CGI) and to provide low-level access to all Microsoft Win32 API functions. Of course, since ISAPI dynamic-link libraries (DLLs) are generally written in a high-level programming language such as C or C++, they are usually more difficult to develop than ASP-based or CGI-based solutions and might not scale as well.

There are two kinds of ISAPI DLLs, which have different uses on IIS:

  • ISAPI extensions:
    Run-time DLLs that can run either in process or out of process on IIS. ISAPI extensions provide extended functionality to IIS.


  • ISAPI filters:
    Used to preprocess packets of data before they enter or leave the IIS main process. An example of an ISAPI filter is the Secure Sockets Layer (SSL) protocol component on IIS 4.



You can create ISAPI extensions easily using the ISAPI Extension Wizard in Microsoft Visual C++.