What split brain is
When a network partition cuts a cluster in two, each side may believe the other is dead and elect its own leader. Now two leaders accept writes independently, and data diverges. This is split brain.
Quorum to the rescue
A quorum requires any decision to be approved by a majority of nodes. With five nodes, a side needs three to act.
- A partition can give a majority to at most one side.
- The minority side cannot reach quorum, so it stops accepting writes.
- This sacrifices availability on the minority side to preserve consistency.
Choosing the right number
Use an odd number of nodes so a clean split always yields a clear majority. Five nodes tolerate two failures while still requiring three to act.
Key idea
Quorum requires a majority to act, so a partitioned cluster lets at most one side make progress, trading minority availability to prevent split brain divergence.