Split-brain.
The radio dies, both halves of the crew promote a captain, and both captains start giving orders.
A cluster with one leader is a calm place: writes go to the boss, everyone else copies along. Then a switch fails, and the cluster is suddenly two groups that cannot hear each other. Each group sees the same evidence, a leader gone silent, and draws the obvious conclusion from where it sits: it is still up over here, or it must be dead.
So each side does the responsible thing and elects a leader. Two leaders now, both legitimate by their own lights, both accepting writes. Your account balance is 40 on one side and 75 on the other, written in good faith by machines following the rules. Split-brain is not a crash — it is a disagreement with no referee.
- 1
Followers take every cue from one leader, which is why there is exactly one version of the truth.
- Hello? …rude.2
A cut looks identical from both sides: everyone over there simply stopped answering.
- Crown fits. Carrying on.3
Neither election broke a rule — each half acted correctly on the half of the picture it could see.
- 4
Each write a side accepts is one more fact the other side will never have heard of.
- 5
Healing the cable is the easy part; both histories are internally consistent and mutually impossible.
- 6
Demand a majority before leading, and the smaller half disqualifies itself on the spot.
Arithmetic as referee
More than half is a clever threshold because two groups can never both clear it. Five nodes split 3–2: the three elect a leader; the two count heads, come up short, and stand down. No coordination is needed across the broken link — each side only has to count its own friends. It is also why clusters come in odd sizes: four nodes can split 2–2 and leave nobody in charge at all.
What the minority gives up
Those two standing-down nodes are healthy machines refusing work, which feels wasteful right up until the first time you untangle a forked dataset by hand. A partitioned system gets exactly one choice: keep a single truth and let part of the system go quiet, or stay fully available and accept two truths to merge later. Split-brain is the name for taking the second option by accident.