What happens when a node fails in a consistent hash ring?
The failed node sits on the ring but no client can reach it. Keys that belonged to that node walk clockwise to the next live node. If the ring had 4 nodes and one dies, roughly 1/3 of keys rehash to one of the 3 survivors. If you use virtual nodes, fewer keys move because they're spread across multiple vnodes.
Answered in
When 87% of Your Cache VanishesConsistent hashing maps nodes and keys to a circle, so adding a server moves only 1/N of keys instead of almost all of them.
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