What are virtual nodes and why do they matter?
A physical node doesn't hash to one point on the ring—it hashes to 100 or 150 (typically 100–500) virtual points scattered around it. Each vnode owns a small arc. When a physical node fails, its vnodes' keys spread across many survivors instead of concentrating on one. Load balances better too: if one physical node hashes unluckily on the ring, its vnodes cover more ground.
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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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