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Quorum Protocol

Quorum Protocol. A scenario. Alice and Bob have 10 friends that they see on a regular basis, but not each friend every day. As she meets them, Alice begins telling these friends something she wants Bob to know. Alice shares the information with 6 friends.

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Quorum Protocol

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  1. Quorum Protocol Dennis Kafura – CS5204 – Operating Systems

  2. A scenario • Alice and Bob have 10 friends that they see on a regular basis, but not each friend every day. • As she meets them, Alice begins telling these friends something she wants Bob to know. Alice shares the information with 6 friends. • Later, Bob – expecting to hear from Alice – begins asking these friends if they have heard from Alice. • How many friends must Bob ask to be guaranteed of receiving the information from Alice? Dennis Kafura – CS5204 – Operating Systems

  3. A scenario • Alice and Bob have N friends that they see on a regular basis, but not each friend every day. • As she meets them, Alice begins telling these friends something she wants Bob to know. Alice shares the information with W friends. • Later, Bob – expecting to hear from Alice – begins asking these friends if they have heard from Alice. • How many friends, R, must Bob ask to be guaranteed of receiving the information from Alice? Dennis Kafura – CS5204 – Operating Systems

  4. Replication and Quorums • Replication improves reliability and performance. • Reliability: increased probability of an available replica • Performance: read most quickly accessible replica • However, availability may be lost • If all replicas must be updated and a replica is unavailable (failure, network partition, overload) • Client must wait until all replicas available • Solution: • reads must obtain R replicas • writes must update W replicas • even if not all N replicas are available • Problem: • inconsistency may arise if R and W are not chosen carefully. Dennis Kafura – CS5204 – Operating Systems

  5. Loss of Consistency W + R  N W: writes R: reads . . . . . . . . . 1 1 1 1 1 1 2 2 2 N Dennis Kafura – CS5204 – Operating Systems

  6. Quorum Consistency W + R N W: writes R: reads . . . . . . . . . 1 1 1 1 1 1 2 2 2 N Dennis Kafura – CS5204 – Operating Systems

  7. Quorum Consistency W + R N W: writes R: reads . . . . . . . . . 1 1 1 1 1 1 2 2 2 2 N Dennis Kafura – CS5204 – Operating Systems

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