The safety life cycle is the series of phases from initiation and specifications of safety requirements, covering design and development of safety features in a safety-critical system, and ending in decommissioning of that system.
The basic concept in building software safety, i.e. safety features in software, is that safety characteristics and behaviour of the software and system must be specified and designed into the system.
The problem for any systems designer lies in reducing the risk to an acceptable level and of course, the risk tolerated will vary between applications.
When a software application is to be used in a safety-related system, then this must be borne in mind at all stages in the software lifecycle.
The process of safety specification and assurance throughout the development and operational phases is sometimes called the ‘safety life cycle’.
The first stages of the life cycle involve assessing the potential system hazards and estimating the risk they pose. One such method is fault tree analysis.
This is followed by a safety requirements specification which is concerned with identifying safety-critical functions (functional requirements specification) and the safety integrity level for each of these functions. The specification may either describe how the software should behave to minimize the risk or might require that the hazard should never arise.
A ‘normal’ process model is then followed with particular attention paid to the validation (inspection, testing etc.) of the system. Part of that validation should be an explicit safety validation activity.
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“The cycle of the machine is now coming to an end. Man has learned much in the hard discipline and the shrewd, unflinching grasp of practical possibilities that the machine has provided in the last three centuries: but we can no more continue to live in the world of the machine than we could live successfully on the barren surface of the moon.”
—Lewis Mumford (1895–1990)