Consensus with Raft — Leader Election & Log Replication
The algorithm that ties leader, log, heartbeat & quorum together
Raft makes a set of replicas agree on an ordered log even as nodes crash — the engine behind etcd, Consul, CockroachDB. Three roles, coordinated by an ever-increasing term (a logical clock); each term has at most one leader.
Leader election
- A follower that hears no heartbeat before its (randomised) timeout becomes a candidate, bumps the term, votes for itself.
- It requests votes; each node grants one vote/term, only if the candidate's log is up-to-date.
- Win a majority (a quorum) → become leader, send heartbeats. Randomised timeouts make split votes rare.
Log replication
- Clients send commands to the leader, which appends to its log (the WAL idea).
- It replicates each entry; once a majority store it, the entry is committed and applied.
- Committed entries survive leader crashes — the election rule guarantees a new leader already has them.
Why no split-brain
Commit needs a majority; two leaders in one term are impossible (one vote per node); a partitioned old leader can't reach a majority, so it commits nothing and steps down on seeing a higher term. Same quorum overlap guarantee: any two majorities share a node.
Takeaways
- Raft = election + log replication, coordinated by monotonic terms.
- “Committed” = stored by a majority — consensus is a quorum write over an ordered log.
- Unifies leader/follower, heartbeat, WAL and quorum into one protocol.
Re-authored from-scratch; diagram hand-authored (SVG) for this guide.
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