How nodes agree on a single value despite failures.
Consensus: Paxos and Raft: how nodes agree on a single value despite failures
choose the failure mode you can explain
5-node cluster tolerates 2 failures.
Consensus protocols let a set of nodes agree on a value even with crashes and message loss. Raft is the more readable cousin of Paxos: elect a leader, replicate the log, and require a majority for every commit.
Raft: leader election (term + heartbeats), log replication (append + commit when majority acks), safety (majority overlaps).
Paxos: equivalent guarantees, harder to teach. Multi-Paxos optimizes for steady-state throughput.
Cost: every commit needs majority; latency floor across the slowest majority node.
etcd cluster of 5 nodes used for service config.