Consensus.

also: consensus algorithm · consensus algorithms · consensus protocol · Raft consensus · Raft algorithm · Raft protocol

Getting a group of nodes to agree on one value or one ordered log, despite failures, as long as a majority is up (Raft, Paxos).

the two-minute refresher

Agree on who leads, which operations apply, and in what order.
Raft: followers expect heartbeats; on timeout one becomes candidate, bumps the term and asks for votes.
Majority votes make a leader; at most one leader per term, and terms act like fencing tokens.
Famous algorithms: Paxos (correct but hard), Raft (built for understandability), Zab (ZooKeeper).

also: consensus algorithm · consensus algorithms · consensus protocol · Raft consensus · Raft algorithm · Raft protocol

Getting a group of nodes to agree on one value or one ordered log, despite failures, as long as a majority is up (Raft, Paxos).

Agree on who leads, which operations apply, and in what order.
Raft: followers expect heartbeats; on timeout one becomes candidate, bumps the term and asks for votes.
Majority votes make a leader; at most one leader per term, and terms act like fencing tokens.
Famous algorithms: Paxos (correct but hard), Raft (built for understandability), Zab (ZooKeeper).