The Impact of Partial Orders among Strict and Probabilistic Quorums on the Data Consistency vs. Operation Availabilities Trade-Off

Abstract

Operation availabilities of pessimistic quorum-based replication strategies are strictly upper-bounded. Probabilistically relaxing their strict consistency notion permits to overcome this bound by introducing probabilistic data replication strategies. These strategies allow the exploitation of the data consistency vs. operation availabilities trade-off. In our approach, we add a certain number of only probabilistically intersecting quorums to the originally strictly intersecting partially ordered quorum system in order to "closely conserve" their original characteristic behavior. As one expects, the number and nature of probabilistic quorums added have a strong impact on the particular trade-off. But furthermore, it turns out that the order in which probabilistic and strict quorums must be probed is also very important. We relate two general strategies of ordering the probabilistic and strict quorums of those probabilistic quorum systems and evaluate their impact on the data consistency vs. operation availabilities trade-off. The evaluation uses Markov chain-based steady state analysis applied to representative instances of probabilistic quorum systems with the adequate partial orderings.