An earlier version of this paper appeared in a chapter in the book “Pattern Languages of Program Design 2 ” ISBN

Norm Kerth published by Addison-Wesley, 1996. This paper describes the Half-Sync/Half-Async pattern, which integrates synchronous and asynchronous I/O models to support both programming simplicity and execution effi-ciency in complex concurrent software systems. In this pat-tern, higher-level tasks use a synchronous I/O model, which simplifies concurrent programming. In contrast, lower-level tasks use an asynchronous I/O model, which enhances ex-ecution efficiency. This pattern is widely used in operating systems such as UNIX, Mach, Windows NT, and VMS, as well as other complex concurrent systems. 1 Intent The Half-Sync/Half-Async pattern decouples synchronous I/O from asynchronous I/O in a system to simplify concurrent programming effort without degrading execution efficiency.