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Consistency

ANY​

  • any server (may not be replica)
  • Fastest: coordinator caches write and replies quickly to client

ALL​

  • all replicas
  • Ensures strong consistency, but slowest

ONE​

  • at least one replica
  • Faster than ALL, but cannot tolerate a failure

QUORUM​

  • quorum across all replicas in all datacenters
  • Majority > 50%
  • Quorums faster than ALL, but still ensure strong consistency
  • Several key-value/NoSQL stores (e.g., Riak and Cassandra) use quorums

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Types of Consistency​

  • Cassandra offers Eventual Consistency