Split Brain

ELI5 - The Vibe Check

Split brain is when a network hiccup cuts a cluster of servers in half, and both halves, unable to talk to each other, each decide they're the boss and keep accepting writes independently. Now you've got two different sources of truth that have quietly diverged, and reconciling them later means somebody's data is going to lose. It's one of the scariest failure modes in distributed systems precisely because everything looks healthy from inside each half.

Real Talk

Split brain occurs when a network partition divides a distributed cluster into isolated groups that each believe they are the sole authoritative leader, leading to conflicting, divergent writes that must later be reconciled or discarded. It's prevented by quorum-based leader election (requiring a majority of nodes to agree, so at most one partition can have enough votes to elect a leader) and by fencing mechanisms that revoke a former leader's ability to write once it's no longer trusted.

When You'll Hear This

"The network partition caused a split brain, both nodes thought they were primary for six minutes." / "Quorum-based election is specifically designed so split brain can't produce two simultaneous leaders."