Comparison
ConsensusvsQuorum
Consensus
a group of nodes has to agree on one value and keep agreeing even when some of them are unreachable or lying about the order.
The protocol family that gets a majority of nodes to agree on an ordered sequence of decisions. It underpins leader election, configuration stores and distributed locks. It requires a majority to be reachable, so a three-node cluster survives one failure and a five-node cluster survives two — and an even number of nodes buys you nothing.
Full entry →Quorum
you require three of the five nodes to acknowledge a write and two to answer a read, so the two sets always overlap.
Requiring acknowledgement from a subset of replicas such that read and write sets are guaranteed to intersect. It gives tunable consistency: raise W for durability, raise R for freshness, keep R + W greater than N for overlap. Quorum reads still return stale data under some failure interleavings, so it is not the same as linearisability.
Full entry →