ISAPI stands for Internet Server Application Programming Interface. ISAPI extensions are modules implemented as plain old Win32 .dll, on which IIS relies to process specific resources. Mappings between ISAPI extensions and files are configured via the IIS snap-in and stored in the IIS metabase, where each file extension can be associated with a particular ISAPI extension, that is, when a request for such a file arrives, IIS handles it to the corresponding ISAPI extension, confident that it will be able to handle it.