Formalising and Optimising Parallel Snapshot Isolation

Modern Internet services often achieve dependability by relying on geo-replicated databases that provide consistency models for transactions weaker than serialisability. We investigate a promising consistency model of Sovran et al.’s parallel snapshot isolation (PSI), which weakens the classical snapshot isolation in a way that allows more efficient geo-replicated implementations. We first give a declarative specification to PSI that does not refer to implementation-level concepts and, thus, allows application programmers to reason about the behaviour of PSI databases more easily. We justify our high-level specification by proving its equivalence to the existing low-level one. Using our specification, we develop a criterion for checking when a set of transactions executing on PSI can be chopped into smaller pieces without introducing new behaviours. This allows application programmers to optimise the code of their transactions so that they get executed more efficiently. We find that our criterion is more permissive than the existing one for chopping serialisable transactions. These results contribute to understanding the complex design space of consistency models for geo-replicated databases.