Mental Model
Redundancy
A reusable way of thinking about reliability by adding backup capacity, alternate paths, or duplicate capability.
softwareinfrastructure
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Redundancy is easy to admire and easy to misuse. The useful question is not "do we have a backup?" It is "will the backup behave independently when the primary fails?"
Redundancy only helps when failure modes are not perfectly correlated. Two replicas in the same failure domain may look redundant on a diagram while failing together in production.