Quorum protocol
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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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Quorum protocol

Quorum Protocol

Dennis Kafura – CS5204 – Operating Systems


A scenario

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


A scenario1

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


Replication and quorums

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


Loss of consistency

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


Quorum consistency

Quorum Consistency

W + R N

W: writes

R: reads

. . .

. . .

. . .

1

1

1

1

1

1

2

2

2

N

Dennis Kafura – CS5204 – Operating Systems


Quorum consistency1

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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