System design interview question
"You run a compute cluster serving production traffic. Design its redundancy: machines fail, racks lose power, availability zones go down, and occasionally a bad deployment takes out everything healthy at once. How much redundancy, arranged how, and what does it cost?"
reasoning about correlated failure and the economics of redundancy. Most candidates can say "run multiple replicas." The discriminating question is which failures are independent — because redundancy only helps against independent failures, and the outages that actually hurt are correlated ones that N+1 capacity does nothing for.
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