Week 2 - Dependability Concepts Flashcards
When is a system said to be dependable?
If it is trustworthy enough for reliance to be justifiably placed on the services it delivers
System
A set of elements that work together to provide a service
System failure
When the services provided by a system deviate from its specification
Error
An erroneous system state that can lead to a failure
Give 4 examples of dependability means
Fault Prevention
Fault Tolerance
Fault Removal
Fault Forecasting
What is a fail-safe system?
A system that becomes safe when it can not operate
What is a fail-operational system?
A system that continues to operate when it fails
What is a fail-secure system?
A system that maintains maximum security when it can not operate
What is a fail-silent system?
A system that stops operating after a failure is detected
Availability
The probability at time t that the system is functioning correctly.
Safety
The extent to which a system can operate without damaging or endangering its environment
Maintainability
A function of time t representing the probability that a failed computer system will be repaired in t time or less
What 2 parts make up a system specification?
Safety
Liveness
If a program satisfies safety only, what is it said to be?
Fail-safe Fault-tolerant
If a program satisfies liveness only, what is it said to be?
Non-masking Fault-tolerant
If a program satisfies safety and liveness, what is it said to be?
(masking) Fault-tolerant
If a program satisfies neither safety or liveness only, what is it said to be?
Fault intolerant
