Epidemia: Variable Consistency for Transactional Cloud Databases

Classic replication protocols running on traditional cluster-based databases are currently unable to meet the ever-growing scalability demands of many modern software applications. Recent cloud-based storage repositories overcome such limitations by fostering availability and scalability over data consistency and transactional support. However, many applications that cannot resign from their transactional nature are unable to benefit from the cloud paradigm. This paper presents Epidemia, a distributed storage architecture featuring a hybrid approach that combines classic database replication with a cloud-inspired infrastructure to provide transactional support and high availability. This architecture is able to offer different consistency levels according to the client demands, thanks to a replication strategy based on epidemic updates in which the replicas of each data partition are organized hierarchically. Additionally, the behavior of a prototype implementation under different workload scenarios is evaluated. Conducted experiments verify that (1) configuration parameters such as the partitioning scheme or the replication protocol play a crucial role on system’s throughput, and (2) the existence of replica hierarchies that are asynchronously updated is able to alleviate the scalability limitations of traditional replicated databases by directing transactions that tolerate a certain staleness in the versions of retrieved data items to these replicas.