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Quorum

The number of copies an operation must reach to count. A write waits for W of the N copies and a read asks R; when R + W > N, every read overlaps the latest acknowledged write.

Taught in Chapter 29: Quorums.

More from the same chapter

• Consensus: A protocol by which nodes agree on a value, such as who the leader is, by majority vote.
• Raft: A consensus protocol: nodes elect one leader per term, and an entry is committed once a majority has stored it.
• Read repair: Fixing a replica that returned an old value by writing the newest one back to it during a read.
• Sloppy quorum: Sending writes to stand-in nodes outside the N while a copy is down. Writes keep working, but reads no longer surely overlap them.