Replica Refresh Strategies in a Database Cluster

Relaxing replica freshness has been exploited in database clusters to optimize load balancing. However, in most approaches, refreshment is typically coupled with other functions such as routing or scheduling, which make it hard to analyze the impact of the refresh strategy itself on performance. In this paper, we propose to support routing-independent refresh strategies in a database cluster with mono-master lazy replication. First, we propose a model for capturing existing refresh strategies. Second, we describe the support of this model in Refresco, a middleware prototype for freshness-aware routing in database clusters. Third, we describe an experimental validation to test some typical strategies against different workloads. The results show that the choice of the best strategy depends not only on the workload, but also on the conflict rate between transactions and queries and on the level of freshness required by queries. Although there is no strategy that is best in all cases, we found that one strategy, ASAUL(0), is usually very good and could be used as default strategy.