Thursday

Concurrency—State Models & Java Programs

Between June 1985 and January 1987, a computerized radiation therapy machine called Therac-25 caused six known accidents involving massive overdoses with follow-on deaths and serious injuries. Although most accidents are systemic involving complicated interactions between various components and actions, and Therac-25 is not an exception in this respect, concurrent programming errors played an vital part in these six accidents. Race conditions between different concurrent actions in the control program resulted in occasional erroneous control outputs. Furthermore, the sporadic nature of the errors caused by faulty concurrent programs contributed to the falter in recognizing that here was a problem. The designers of the Therac-25 software seemed largely unaware of the principles and practice of concurrent programming.

The wide acceptance of Java with its in-built concurrency constructs means that concurrent programming is no longer restricted to the underground of programmers caught up in operating systems and embedded real-time applications. Concurrency is helpful in a wide range of applications everywhere responsiveness and throughput are issues. While most programmers are not engaged in the implementation of safety critical systems such as Therac-25, increasing numbers are using concurrent programming constructs in less esoteric applications. Errors in these applications and systems may not be frankly life-threatening but they adversely affect our quality of life and may have severe financial implications. An understanding of the principles of concurrent programming and an appreciation of how it is practiced are an essential part of the education of computing science undergraduates and of the background of software engineering professionals. The pervasive nature of computing and the Internet makes it also an vital topic for those whose primary activity may not be computing but who write programs none the less.

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