What is split brain in a clustered system, and how does quorum prevent it?

Split brain is when a network partition leaves two halves of a cluster each believing they are the leader, so both accept writes and the data diverges. Quorum prevents it by requiring a majority, N/2 plus 1, to elect a leader or commit a write, and only one side of a partition can hold a majority. Use odd node counts, fencing tokens, and never let a minority partition serve writes.

Say you have five nodes and a switch fails so you end up with a group of three and a group of two that cannot see each other. Without quorum, both groups notice the leader is gone, both elect their own, and now you have two sources of truth accepting conflicting writes, which is genuinely painful to reconcile afterward. Quorum means you need three of five to elect or commit, so the group of two simply cannot make progress and has to reject writes. That is the trade: you buy consistency by giving up availability on the minority side. Two things I always check in practice. One, use odd numbers, because six nodes still need four for quorum and tolerate the same two failures as five while costing more. Two, fencing. The old leader may not know it was demoted, so you need monotonic fencing tokens or storage level locks, otherwise a paused process wakes up and writes to a volume it no longer owns.