#6 most askedVery commonHardConsistent hashing

Consistent hashing

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Why does hash(key) % N break when you add a cache server, and how does consistent hashing fix it?

🧭 Adding a node to the hash ring

Key user:42

Hash ring

Node A

Node B

New node D

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const hash = (s) => [...s].reduce((h, c) => (h * 31 + c.charCodeAt(0)) >>> 0, 7) % 1000;
const keys = Array.from({ length: 1000 }, (_, i) => "user:" + i);
const modMoved = keys.filter((k) => hash(k) % 4 !== hash(k) % 5).length;
const ring = (nodes) => (k) => nodes.find((n) => n >= hash(k)) ?? nodes[0];
const before = ring([100, 350, 600, 850]), after = ring([100, 350, 600, 725, 850]);
const ringMoved = keys.filter((k) => before(k) !== after(k)).length;
console.log(`4→5 servers: modulo moved ${modMoved / 10}% of keys, consistent hashing moved ${ringMoved / 10}%`);

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