The dependability of a computer system is a property of the system that reflects its trustworthiness. Trustworthiness here essentially means the degree of confidence a user has that the system will operate as they expect, and that the system will not ‘fail’ in normal use. It is not meaningful to express dependability numerically.
There are four principal dimensions to dependability, as shown in Figure.
1. Availability: The availability of a system is the probability that it will be up and running and able to deliver useful services to users at any given time.
2. Reliability: The reliability of a system is the probability, over a given period of time, that the system will correctly deliver services as expected by the user.
3. Safety Informally: The safety of a system is a judgment of how likely it is that the system will cause damage to people or its environment.
4. Security Informally: The security of a system is a judgment of how likely it is that the system can resist accidental or deliberate intrusions.
