Performance of program modification techniques that ensure serializable executions with snapshot isolation DBMS

Snapshot Isolation (SI) is a multiversion concurrency control that has been implemented by several open source and commercial database systems (Oracle, PostgreSQL, Microsoft SQL Server). The main feature of SI is that a read operation does not block a write operation and vice versa, which allows higher degree of concurrency than traditional two-phase locking. SI prevents many anomalies that appear in other isolation levels, but it still can result in non-serializable executions, in which database integrity constraints can be violated. Several techniques are known to modify the application code in order to ensure that every execution is serializable on engines running SI. Each modification technique introduces some conflicts, and so prevents certain interleavings, without altering the functionality of each application program. In following one of the possible techniques of application modification, there are choices that must be be made, between different conflicts where interleavings are affected. This paper investigates the performance impact of the different techniques, and different choices of modifications, that can ensure that all executions are serializable. We here propose one technique called ’External Lock Manager’ (ELM) which introduces conflicts in a separate lock-manager object rather than through SQL statements in the DBMS. We find that ELM performs well under a range of situations, with peak performance close to SI no matter which conflicts are chosen to be introduced.