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Lecture Software engineering (7/e): Chapter 3 – Ian Sommerville

The learning objectives for this chapter include: To explain what is meant by a critical system where system failure can have severe human or economic consequence; to explain four dimensions of dependability - availability, reliability, safety and security; to explain that, to achieve dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure. | Critical Systems Objectives To explain what is meant by a critical system where system failure can have severe human or economic consequence. To explain four dimensions of dependability - availability, reliability, safety and security. To explain that, to achieve dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure. Topics covered A simple safety-critical system System dependability Availability and reliability Safety Security Critical Systems Safety-critical systems Failure results in loss of life, injury or damage to the environment; Chemical plant protection system; Mission-critical systems Failure results in failure of some goal-directed activity; Spacecraft navigation system; Business-critical systems Failure results in high economic losses; Customer accounting system in a bank; System dependability For critical systems, it is usually the case that the most important system property is the dependability of the system. The . | Critical Systems Objectives To explain what is meant by a critical system where system failure can have severe human or economic consequence. To explain four dimensions of dependability - availability, reliability, safety and security. To explain that, to achieve dependability, you need to avoid mistakes, detect and remove errors and limit damage caused by failure. Topics covered A simple safety-critical system System dependability Availability and reliability Safety Security Critical Systems Safety-critical systems Failure results in loss of life, injury or damage to the environment; Chemical plant protection system; Mission-critical systems Failure results in failure of some goal-directed activity; Spacecraft navigation system; Business-critical systems Failure results in high economic losses; Customer accounting system in a bank; System dependability For critical systems, it is usually the case that the most important system property is the dependability of the system. The dependability of a system reflects the user’s degree of trust in that system. It reflects the extent of the user’s confidence that it will operate as users expect and that it will not ‘fail’ in normal use. Usefulness and trustworthiness are not the same thing. A system does not have to be trusted to be useful. Importance of dependability Systems that are not dependable and are unreliable, unsafe or insecure may be rejected by their users. The costs of system failure may be very high. Undependable systems may cause information loss with a high consequent recovery cost. Development methods for critical systems The costs of critical system failure are so high that development methods may be used that are not cost-effective for other types of system. Examples of development methods Formal methods of software development Static analysis External quality assurance Socio-technical critical systems Hardware failure Hardware fails because of design and manufacturing errors or because components have .