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quorum
A majority must agree before a write is real. Two nodes out of three is enough. One node must not be.
What is quorum?
A quorum is the minimum number of nodes that must confirm an operation. With 2f+1 nodes you tolerate f failures. A write quorum and a read quorum that overlap means a read sees the latest write. Split brain is what you get when two sides both think they have quorum.
Why does quorum matter when vibe coding?
Models set replicas to 2 and accept a write when either says yes. A partition then creates two truths. Say the numbers: N, and how many must ack.
How do you do quorum?
Pick an odd replica count. Writes ack a majority. Reads, if they must be fresh, also hit a majority or a leader. Refuse to elect two leaders.
How do you ask a model for quorum?
Use a quorum for (store): N=(odd number). Acknowledge a write only after a majority. Do not accept a write from a single node if a partition could create a second writer.
What goes wrong with quorum?
Majority of a changing membership you never recompute. A dead node still counts, so you can never form a quorum. Membership is part of the protocol.