You depend on a subject with critical functionality whose absence or malfunction would lead to not managable problem in the overall system.
The absence or malfunction of the subject leads to a malfunction of the overall system.
“Use” more than one copy of the subject and implement a mechanism which allows to switch from one/all to one/the remaining subjects in case of the malfunction of one/many subject(s).
Use cases:
  • employees with key skills/in key positions
  • safety/availability critical software and systems
Resulting context:
The malfunction or absence of up to n-1 of the n copies of the subject do not result in a malfunction of the overall system. The probability of the system failing (probability of all redundant copies failing) \(p_{system}\) is \(p_{system} = \prod \limits_{i=1}^{n}p_{i}\) with \(n\) number of subjects and \(p_{i}\) probability of subject i failing.