From defects to failures: a view of dependable computing

A unified framework and terminology for the study of computer system dependability is presented. Impairments to dependability are viewed from six abstraction levels. It is argued that all of these levels are useful, in the sense that proven dependability procurement techniques can be applied at each level, and that it is beneficial to have distinct, precisely defined terminology for describing impairments to and procurement strategies for computer system dependability at these levels. The six levels in the proposed framework are:1. Defect level or component level, dealing with deviant atomic parts.2. Fault level or logic level, dealing with deviant logic values or path selections.3. Error level or information level, dealing with deviant internal states.4. Malfunction level or system level, dealing with deviant functional behavior.5. Degradation level or service level, dealing with deviant performance.6. Failure level or result level, dealing with deviant outputs or actions.Briefly, a hardware or software component may be defective (perfect hardware may also become defective due to wear and aging). Certain system states expose the defect , resulting in the development of a logic-level fault . Information flowing within a faulty system may become contaminated, leading to the presence of an error . An erroneous system state may result in a subsystem malfunction . Automatic or manually controlled reconfiguration can isolate or bypass the malfunctioning subsystem but may lead to a degraded performance or service. Serious performance degradation may lead to a result-level system failure when untrustworthy or untimely results are produced. Finally, a failed computer system can have adverse effects on the larger societal or corporate system into which it is incorporated.