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Abstract. Modern Internet services often achieve scalability and availability by relying on large-scale distributed databases that provide consistency models for transactions weaker than serialisability. We investigate the classical problem of transaction chopping for a promising consistency model in this class-parallel snapshot isolation (PSI), which weakens the classical snapshot isolation to allow more efficient large-scale implementations. Namely, we propose a criterion for checking when a set of transactions executing on PSI can be chopped into smaller pieces without introducing new behaviours, thus improving efficiency. We find that our criterion is more permissive than the existing one for chopping serialisable transactions. To establish our criterion, we propose a novel declarative specification of PSI that does not refer to implementation-level concepts and, thus, allows reasoning about the behaviour of PSI databases more easily. Our results contribute to building a theory of consistency models for modern large-scale databases.

Snapshot isolation (SI) is a widely used consistency model for transaction processing, implemented by most major databases and some of transactional memory systems. Unfortunately, its classical definition is given in a low-level operational way, by an idealised concurrency-control algorithm, and this complicates reasoning about the behaviour of applications running under SI. We give an alternative specification to SI that characterises it in terms of transactional dependency graphs of Adya et al., generalising serialisation graphs. Unlike previous work, our characterisation does not require adding additional information to dependency graphs about start and commit points of transactions. We then exploit our specification to obtain two kinds of static analyses. The first one checks when a set of transactions running under SI can be chopped into smaller pieces without introducing new behaviours, to improve performance. The other analysis checks whether a set of transactions running under a weakening of SI behaves the same as when running under SI.