Correctness of Parallel Executions in Multidatabase Systems Ruled by Strict 2 Phase Locking

This paper addresses the problem of transaction management in multidatabase systems where the participating local DBMSs serialize transactions by 2 Phase-Locking (2PL) and synchronize their commit actions through a 2 Phase-Commit protocol (2PC). These DBMSs represent the majority of commercial relational and objectoriented DBMSs. We demonstrate that if local DBMSs support strict 2PL optimization (i.e., relax the read locks during the prepare phase of the 2PC protocol), then schedules of parallel global transactions may become nonglobally serializable. X/Open DTP protocol avoids the problem by adding a blocking synchronization phase at transaction end, thereby loosing the whole benefit of strict 2PL. This paper proposes two strategies to preserve the benefit of strict 2PL while ensuring correct executions.