An Adaptable Constrained Locking Protocol for High Data Contention Environments

Multiversion concurrency control schemes are often limited in their practicability due to their storage requirements for multiple versions of the data. However, a class of multiver-sion schemes utilize only the versions, maintained for the purpose of recovery, to improve the concurrency by allowing the concurrent execution of "non conflicting" read-write lock requests on different versions of data in an arbitrary fashion. A transaction that accesses a data item version which is later diagnosed to lead to a incorrect execution, is aborted. This act is reminiscent of the validation phase in the optimistic concurrency schemes. Various performance studies suggest that these schemes perform poorly in high data contention environments where the excessive transaction aborts result, due to the failed validation. We propose an adaptable constrained two version two phase locking (C2 V2PL) 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 hence, must be either rejected or blocked. This eliminates the need for a separate 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 in this state further reduces the duration for which locks are held by a transaction. The C2V2PL scheduler adapts to the low data contention environments by accepting the lock requests that have failed the specific constraints but contrary to the assumption will not lead to an incorrect execution. Thus improving the performance due to reduced transaction aborts in this conservative state.