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ISAPI (Internet Server Application Program Interface) is a set of Windows program calls that let you write a Web server application that will run faster than a common gateway interface (CGI) application. A disadvantage of a CGI application (or "executable file," as it is sometimes called) is that each time it is run, it runs as a separate process with its own address space, resulting in extra instructions that have to be performed, especially if many instances of it are running on behalf of users. Using ISAPI, you create a dynamic link library (DLL) application file that can run as part of the Hypertext Transport Protocol (HTTP) application's process and address space. The DLL files are loaded into the computer when HTTP is started and remain there as long as they are needed; they don't have to be located and read into storage as frequently as a CGI application.
Existing CGI applications can be converted into ISAPI application DLLs without having to rewrite their logic. However, they do need to be written to be thread-safe so that a single instance of the DLL can serve multiple users.
A special kind of ISAPI DLL is called an ISAPI filter, which can be designated to receive control for every HTTP request. You can create an ISAPI filter for encryption or decryption, for logging, for request screening, or for other purposes.
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http://www.codeproject.com/KB/aspnet/ASPDOTNETPageLifecycle.aspx