Dependability

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'.

Requirement	Description
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 vs. Availability

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.