ON CONCURRENCY CONTROL FOR REAL-TIME DATABASE SYSTEMS

Abstract

Research on real-time database systems (RTDBS) has received much attention in recent years. RTDBS are generally defined as database systems m which the transactions have constraints on their completion times, expressed as then- deadlines. Real-time database systems can be classified as hard, firm or soft based on the effect ofmissing the transaction deadlines. In hard RTDBS, missing a deadlme can potentially result in a catastrophe. In firm RTDBS, transactions impart no value to the system once their deadlmes expire but there is no catastrophic consequences. In soft RTDBS, transactions have some values even after their deadlines are missed, which may drop to zero at certain point past the deadline. Concurrency control is one of the main issues in the study of traditional databases and RTDBS, a comparative study of the issues of concurrency control for traditional databases including various techniques and the requirements and implications for extending those techniques to real-tune database systems has first been presented mthis thesis. Though the prices of hardware components are dramatically dropping, very little work has been done to examine the performance of concurrency control schemes under different availability of system resource m RTDBS. We capture the main elements of database environment including both transactions and physical resources for storing and processing the workload and the databases (CPUs and disks). Different resource-related assumptions are explored and the question of which assumption plays a significant role in improving the system performance is addressed. We first evaluate the performance of OCC and some of its variants under the condition of limited system resources mfirm realtime database system. The results show that as the workload mcreases, the effect of resource contention dominates that of data contention in determinmg the scheme's performance. To eliminate the effect of system resource contention, an unlimited system resources situation is simulated, since m RTDBS, functionality and meetmg the timing constraints of real-time transactions rather than cost is often the driving consideration. The performance of the OCC and its variants while assuming the databases as memory resident is investigated. Studying a memory resident system is important since memory prices are steadily dropping, memory sizes are growing and memory residence becomes less of a restriction. To isolate the effect of system resource contention under the assumption of memory resident databases, we also perform this experiment with unlimited system resources. The performance analysis is contmued by studying the schemes in experiments where the database is disk resident. In these experiments emphasis is placed on how I/O operations impact the performance. Next, the optimistic concurrency control schemes integrated with the virtual run policy for first-run transactions marked for restart using the retention effect of the system buffer are developed and their performances are compared with optimistic concurrency control in firm real-time database systems and it is found that this technique can help the schemes in achieving better performance with moderate system resources availability. The performance of optimistic concurrency control is also evaluated in soft real time database systems under different resource-related assumptions. Two cases of soft RTDBS are considered. In the first case, all transactions are allowed to complete with the assumption that there is always some decreasing value a transaction can impart to the system past its deadline. In the second case, transactions have some decreasing value even after their deadlines which drops to zero at certain point past the deadline. Also for both in cases of soft RTDBS, we have conducted the experiments with the policy of raising the priority of deadline-missed transactions to help them in completing with the smallest possible tardy time so that they can impart maximum possible values past their deadline to the soft real-time database system. The performance analysis of lockmg based schemes (pessimistic) using 2PL scheme as the underlying technique m real-tune database systems under different resourcerelated conditions is discussed and some modification to the conflict resolution mechamsm of the locking schemes is proposed. The sensitivity of performance with respect to some key parameters have been studied and the results are discussed. Also we mvestigate the performance gainbyemploying the pre-emption scheduling policy of theCPU An integrated concurrency control mechamsm which uses optimistic concurrency control for first-run transactions and static locking for second-run transactions and guarantees that a transaction can complete in at most two runs or can have at most one restart, is developed to integrate the benefits of the pessimistic and optimistic approaches for real-time database systems and its performance is compared with real-tune locking based schemes. The results clearly bring out the superiority of the hybrid scheme over other lockmg based schemes. Finally, we give a comparative study of optimistic, pessunistic (lockmg) and the hybrid schemes from performance pomt of view m real-tune database systems.