Asynchronous I/O operations make the most efficient use of the CPU time available to IIS. Because I/O operations often block threads—especially network I/O operations—asynchronous operations are often the most appropriate choice, both for reading and writing. IIS key technologies provided by Windows 2000, including I/O completion ports and robust thread pools, to provide a wide range of asynchronous I/O support.
In general, all asynchronous operations in IIS behave similarly: First your ISAPI extension designates an asynchronous callback function, then your extension calls the appropriate asynchronous read or write function provided by IIS. When IIS completes the read or write operation, IIS calls the asynchronous callback function to allow your extension to perform further processing or cleanup.
For more detailed information and examples, see Reading and Writing Asynchronously, and the Synchronous and Asynchronous I/O ISAPI extension sample.