Split brain.

also: split-brain

Two 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.

also: split-brain

Two nodes both believing they are the leader, for example after a network partition, so both accept conflicting writes.

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.