Dependability, Reliability, and Availability
Dependability is the quality of delivered service such that reliance can justifiably be placed on this service.
Reliability is a measure of the continuous service accomplishment(or, equivalently, of the time to failure) from a reference point.
mean time to failure(MTTF) is a reliability measure. annual failure rate(AFR) is the percentage of devices that would be expected to fail in a year for a given MTTF. Service interruption is measured as mean time to repair(MTTR). Mean time between failures(MTBF) is simply the sum of MTTF and MTTR.
Availability is a measure of service accomplishment with respect to the alternation between the two states of accomplishment and interruption.
Availability = MTTF / MTBF
3 ways to improve MTTF:
- Fault avoidance: preventing fault occurrence by construction.
- Fault tolerance: using redundancy to allow the service to comply with the service specification despite faults occurring, which applies primarily to hardware faults.
- Fault forecasting: predicting the presence and creation of faults, which applies to hardware and software faults, allowing the component to be replaced before it fails.