Snapshot isolation and integrity constraints in replicated databases

Abstract

Database replication is widely used for fault tolerance and performance. However, it requires replica control to keep data copies consistent despite updates. The traditional correctness criterion for the concurrent execution of transactions in a replicated database is 1-copy-serializability. It is based on serializability, the strongest isolation level in a nonreplicated system. In recent years, however, Snapshot Isolation (SI), a slightly weaker isolation level, has become popular in commercial database systems. There exist already several replica control protocols that provide SI in a replicated system. However, most of the correctness reasoning for these protocols has been rather informal. Additionally, most of the work so far ignores the issue of integrity constraints. In this article, we provide a formal definition of 1-copy-SI using and extending a well-established definition of SI in a nonreplicated system. Our definition considers integrity constraints in a way that conforms to the way integrity constraints are handled in commercial systems. We discuss a set of necessary and sufficient conditions for a replicated history to be producible under 1-copy-SI. This makes our formalism a convenient tool to prove the correctness of replica control algorithms.