Concurrency: State Models and Java Programs

Chapter 1. Introduction

Between June 1985 and January 1987, a computerized radiation therapy machine called Therac-25 caused six known accidents involving massive overdoses with resultant deaths and serious injuries. Although most accidents are systemic involving complex interactions between various components and activities, and Therac-25 is not an exception in this respect, concurrent programming errors played an important part in these six accidents. Race conditions between different concurrent activities 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 delay in recognizing that there 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 minority of programmers involved in operating systems and embedded real-time applications. Concurrency is useful in a wide range of applications where 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 directly life-threatening but they adversely affect our quality of ...