Why getting a group of machines to agree on a single value is the root problem under etcd, Consul, Kafka, and every database you run — leader election, quorums and majorities, the FLP impossibility result in plain terms, Paxos vs Raft and why Raft won on understandability, log replication, split-brain and fencing, and how to reason about a cluster that has lost its leader.
Raft
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Consensus and Coordination: Raft, Paxos, and Why Agreement Is Hard