A dynamic-link library (DLL) that is loaded into Internet Information Services (IIS) when it starts and that remains in memory until it stops.
ISAPI filters provide Web servers such as IIS with the ability to preprocess or postprocess information sent between the client and server, and they have no equivalent in the Common Gateway Interface (CGI) scheme.
ISAPI filters receive special filter event notifications and respond based on these notifications. You use ISAPI filters for such tasks as custom authentication, encryption, and compression schemes or for updating logging statistics on the Web server.
ISAPI filters are generally called for every Uniform Resource Locator (URL) that the Web server processes instead of being explicitly invoked by a URL, as are ISAPI extensions.
Custom ISAPI filters can be designed by third-party developers for such tasks as