# Redundancy¶

Context:
You depend on a subject with critical functionality whose absence or malfunction would lead to not managable problem in the overall system.
Problem:
The absence or malfunction of the subject leads to a malfunction of the overall system.
Solution:
“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.