An adaptable constrained locking protocol for high data contention environments: correctness and performance

Multiversions of data are used in database systems to increase concurrency and to provide efficient recovery. Data versions improve the concurrency by allowing the concurrent execution of "non-conflicting" read-write lock req uests on different versions of data in an arbitrary fashion. A transaction that accesses a data item version, which later diagnosed to lead to an inco rrect execution, is aborted. This act is reminiscent of the validation phas e in the optimistic concurrency control schemes. Various performance studie s suggest that these schemes perform poorly in high data contention environ ments where the excessive transaction aborts result due to the failed valid ation. We propose an adaptable constrained two-version two-phase locking (C 2V2PL) scheme in which these "non-conflicting" requests are allowed only in a constrained manner. C2V2PL scheme assumes that a lock request failing to satisfy the specific constraints will lead to an incorrect execution and h ence, must be either rejected or blocked. This eliminates the need for a se parate validation phase. When the contention for data among the concurrent transactions is high, the C2V2PL scheduler in aggressive state rejects such lock requests. The deadlock free nature of C2V2PL scheduler adapts to the low data contention environments by accepting the lock request that have fa iled the specific constraints but contrary to the assumption, will not lead to an incorrect execution. Thus, C2V2PL scheme improves the potential conc urrency due to reduced transaction aborts in this conservative state. We ha ve compared performance of our scheme with other lock-based concurrency con trol schemes such as two phase locking, Wait-depth locking and Optimistic l ocking schemes. Our results show increase in throughput and reduced transac tion-abort-ratio in case of C2V2PL scheme. (C) 2000 Elsevier Science B.V. A ll rights reserved.