Engineering by Software: System Behaviours as Components
Software engineering means developing software for a purpose. For systems that interact with a physical problem world, the proximate purpose is a desired behaviour of that problem world: the fundamental engineering task is to develop the system behaviour and specify the software that will evoke it. This chapter sketches a discipline for this task, in which behaviours are always understood as the result of interactions of a software machine and a physical problem world. Complex behaviour is designed in terms of simple constituent behaviours. The running behaviour instances form a dynamic tree in which control of the tree is exercised by the software machines at the nodes.
Some salient concepts, principles, practices and concerns of this discipline are discussed. After an introductory section, the development of a simple behaviour is clarified, and the iterative relationship between behaviour and requirements is illustrated. Requirements state the benefits expected from the system behaviour. An abstract goal behaviour is proposed, intended to satisfy the requirements. After elaboration into a concrete behaviour of the physical problem world, it is again evaluated against the requirements. The nature and development of complex behaviour is discussed. Development is partly top-down and partly bottom-up: top-down development decomposes a well-understood function into subfunctions; bottom-up development combines disparate functions springing from distinct requirements. In both, the identification and design of constituent behaviours is distinguished from their combination into a complex whole. The chapter ends with some general observations on the principles embodied in the approach described.
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