Why do consensus clusters (etcd, ZooKeeper) use 3 or 5 nodes rather than 4?

Options

• ✓ 4 nodes need a majority of 3, so they survive only 1 failure, the same as 3 nodes, with more traffic Correct
• ✗ An even number of nodes can never elect a leader
• ✗ 4 nodes would need two leaders
• ✗ Odd numbers make hashing faster

Why

Adding a fourth node raises the majority from 2 to 3 without raising the number of failures tolerated. It also adds one more node to every write.

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