Availability · Multiple choice

The math said two replicas give about 99.9999%, but both went down together. Which assumption broke?

medium

Options

That the two replicas fail independently Correct
That availabilities multiply in series
That the replicas use the same software version
That a replica's availability is under 100%

Why

Shared racks, power, networks, zones, configuration pushes and the same bug can take down both at once. Spread replicas over failure domains and roll out changes gradually so failures stay independent.

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