What is ISAPI ?

October 28, 2007 · Filed Under ASP.Net Interview Questions, Placement Questions 



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Microsoft introduced an alternative to CGI, the Internet Server Application Programming Interface (or ISAPI). ISAPI addresses one of the most limiting features of CGI applications. Each time a client requests the execution of a CGI application, the web server executes a separate instance of the application, sends in the user’s requesting information, and serves the results of the CGI application’s processing to the client. The problem with this approach is that a separate CGI application is loaded for each request. This can be quite a drain on the server’s resources if there are many requests for the CGI application.

ISAPI alleviates this problem by relying on dynamic link libraries (DLLs). Each ISAPI application is in the form of a single DLL that is loaded into the same memory space as the web server upon the first request for the application. Once in memory, the DLL stays in memory, answering user requests until it is explicitly released from memory. This increased efficiency in memory usage comes at a cost. All ISAPI DLLs must be thread-safe so that multiple threads can be instantiated into the DLL without causing problems with the application’s function. ISAPI applications are normally faster than their equivalent CGI applications because the web server does not have to instantiate a new application every time a request is made. Once the ISAPI application DLL is loaded into memory, it stays in memory. The web server does not need to load it again

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