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2 Requirements
2.1 Dependable Systems
Hazards to systems are a fact of life. So are faults. Yet we want our systems to be dependable. A system is dependable when it is trustworthy enough that reliance can be placed on the service that it delivers [Carter 82]. For a system to be dependable, it must be available (e.g., ready for use when we need it), reliable (e.g., able to provide continuity of service while we are using it), safe (e.g., does not have a catastrophic consequence on the environment), and secure (e.g., able to preserve confidentiality) [Laprie 92].
Although these system attributes can be considered in isolation, in fact they are interdependent. For instance, a system that is not reliable is also not available (at least when it is not operating correctly). A secure system that doesn't allow an authorized access is also not available. An unreliable system to control nuclear reactors is probably not a safe one either.
A Conceptual Framework for Systems Fault Tolerance - 30 MAR 95
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