Why it matters
Consensus is the foundation of every strongly-consistent distributed system. Raft's clear structure (leader election + log replication + safety) makes it teachable and implementable. This is why it displaced Paxos as the practical default.
The architecture
Roles: Leader (accepts client writes, replicates), Followers (accept writes from leader, respond to heartbeats), Candidates (during election).
Terms: monotonically increasing numbers identifying leader epochs. Every message includes the sender's term.
How it works end to end
Leader election: on heartbeat timeout, follower becomes candidate, increments term, votes for itself, requests votes from others. Majority (N/2 + 1) grants leadership.
Log replication: leader accepts client commands, appends to log, sends AppendEntries to followers. Followers append and ack. Once majority acks, leader commits and applies.
Safety: Raft guarantees committed entries are never overwritten. Leader completeness invariant: any leader for a term contains all committed entries from previous terms.