1. Dennis Kafura – CS5204 – Operating Systems
Quorum Protocol
Dennis Kafura – CS5204 – Operating Systems

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

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

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