Replication & Quorums

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Quorums

  • You need overlap
  • Overlap gives you consistency
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The rules

  • Read/Write: R + W > N
  • Write/Write: W + W > N
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Read Repair

  • Read from R
  • Return newest
  • Repair stale replicas
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Quorum Read Plan

  • Pick value + repair set
  • Check safety (R + W > N)
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Hinted Handoff

  • Write to a fallback if replica is down
  • Store a hint
  • Replay later
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