Serializable executions with Snapshot Isolation and two-phase locking: Revisited

Snapshot Isolation (SI) is a concurrency control mechanism that has been implemented by several commercial and open resources platforms. However, under SI, a set of program may experience a non-serializable execution, in which database integrity constraints can be violated. An elegant approach from Fekete (2005) shows how to guarantee serializable execution on platforms that offer both SI and traditional two-phase locking (2PL) concurrency control, by running some transactions (pivots) with 2PL and the rest at SI. While Fekete's Pivot 2PL technique performs better than running all transactions at 2PL, it often loses much performance compared to SI for all transactions. In this paper we identify causes that harm performance of Pivot 2PL, and we propose an improved approach, called Pivot Ordered2PL, in which a few transactions are rewritten (without changing their functionality). We evaluate Pivot Ordered2PL and find it ensures serializable execution with performance close to that of SI.