Tunable with Quorum

• Vector clock

• Tunable consistency model - Quorum NWR

Vector clock

• Vector clock: Published by Lesie Lamport in 1978. Time, Clocks and the Ordering of Events in a Distributed System

• Clock synchronization: UMass course

• Why vector clocks are easy

• Why vector clocks are hard

Tunable consistency model - Quorum NWR

• Dynamo DB / Cassandra

• Quorum NWR Definition:

  • N: The number of replicas

  • W: A write quorum of size W. For a write operation to be considered as successful, write operation must be acknowledged from W replicas

  • R: A read quorum of size W. For a read operation to be considered as successful, read operation must be acknowledged from R replicas

• If W+R > N, could guarantee strong consistency because there must be at least one overlapping node that has the latest data to ensure consistency

• Typical setup:

  • If R = 1 and W = N, the system is optimized for a fast read

  • If R = N and W = 1, the system is optimized for a fast write

  • If W + R > N, strong consistency is guaranteed (Usually N = 3, W = R = 2)