Two brains, one body
When a network partition splits a cluster, each side may believe the other died and elect its own leader. Now two leaders accept writes independently. This split brain lets the partitions diverge, and merging them later means choosing which conflicting writes to throw away.
The resolver
A split brain resolver is a rule that decides, after a partition, which side may keep operating and which must stop. The most common rule is a majority quorum.
- A side may stay active only if it holds more than half the nodes.
- The minority side steps down, refusing writes.
- With an odd total, at most one side can ever have a majority.
Because only one side can hold a majority, only one side keeps a leader. The other halts rather than risk divergence.
Beyond simple majority
Some clusters add tie breakers for even sized clusters, like a designated node or an external witness. The principle holds, exactly one partition is allowed to proceed and the rest deliberately stop, trading availability for consistency.
Key idea
A split brain resolver prevents two leaders during a partition by letting only the majority side stay active, sacrificing availability on the minority side to protect consistency.