Split brain.
also: split-brainTwo nodes both believing they are the leader, for example after a network partition, so both accept conflicting writes.
the two-minute refresher
- A partition isolates the primary; the replicas promote a new one; the old one keeps taking writes.
- Now two primaries diverge, and merging them later is painful or impossible.
- Fencing stops the old leader: STONITH, revoked storage or network access, leases, fencing tokens.
- Requiring a majority (quorum) to lead means at most one side can win.