Split brain

Two nodes both believe they are the leader, usually after a network partition, and accept conflicting writes. Majority quorums and fencing tokens prevent it.

Where it shows up in real systems

  • Coordination service

    Runs Raft or ZAB on an odd number of nodes: three nodes survive one failure, five survive two. Use it for small, critical data, never as a general database.

    In production: etcd, Apache ZooKeeper, Consul KV

See it break, then fix it

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