Non-blocking ROWA Protocols Implement GSI Using SI Replicas

The concept of Generalized Snapshot Isolation (GSI) has been recently proposed as a suitable extension of conventional Snapshot Isolation (SI) for replicated databases. In GSI, transactions may use older snapshots instead of the latest snapshot required in SI, being able to provide better performance without significantly increasing the abortion rate when write/write conflicts among transactions are low. Its authors also state that GSI is needed because there is no non-blocking implementation of SI in asynchronous systems, even when databases never fail, but they do not prove such statement. We justify such property for ROWA (Read One, Write All) protocols in this paper by using the equivalence between SI-schedules. Additionally, we show and prove that if a replication protocol using SI replicas provides global atomicity and commits update transactions in the same order at all sites then it provides GSI. This sufficient condition prevents the usage of some mechanisms that exclusively order write/write conflicting transactions because they do not guarantee GSI.