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A component makes available services to its clients. So it can make them available, the component may need services from other components -> one component may depend on another component.
Component C depends on C' if the correctness of C depends on the correctness of C'.
Availability
Readiness to be used.
Dependability
Continuous availability of a service.
Security
Low catastrophy probability.
Maintenance
How easy it is to repair the system.
Reliability R(t) of component C.
The conditional probability that C is working correctly during [0,t[ knowing that C was working correctly at t=0.
Additional metrics:
Mean Time To Failure (MTTF): Average time until a component fails.
Mean Time To Repair (MTTR): Average time necessary to repair a component.
Mean Time Between Failures (MTBF): MTTF + MTTR
Availability D(t) of component C.
Average fraction of time between [0,t[ when C has been working.
Long-term availability D: D(∞).
D = MTTF / MTBF = MTTF / (MTTF + MTTR)
Talking about reliability and availability only makes sense when we have a clear notion of what is a failure.
