Relaxing Correctness Criteria in Database Replication With 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. We study and formally proof a sufficient condition that replication protocols with SI replicas following the deferred update technique must obey to achieve GSI. They must provide global atomicity and commit update transactions in the very same order at all sites. However, as this is a sufficient condition, it is possible to obtain GSI by relaxing certain assumptions about the commit ordering of certain update transactions.