Use of portable computers with a wireless connection to distributed databases will become as popular as the use of mobile phones in the near future. The rapidly advancing technology in this area initiates new research areas and challenging research questions recently. One of the new areas is to support real-time database systems with wireless network. In this paper, we have built a model with sufficient details of a wireless distributed real-time database system (WDRTDBS) and have performed simulation experiments to identify the effect of wireless bandwidth, which is one of the most scarce resources in a wireless environment, on the performance of distributed real-time database systems. Through the experiments, we are trying to figure out the criterion for building efficient wireless DRTDBS in terms of performance. The simulation results reveal that the call duration, which may lead to call blocking and prolonged call waiting, causes much impact on the performance of the DRTDBS in terms of resource contention and transaction deadline missing.
IntroductionThe fast growing technology supporting wireless computing made a lot of applications running in a wireless environment possible. In near future, it may be very common to carry a portable computer with a wireless connection to the information network. As a result, it will not be far from reality for a mobile client (MC) to issue a transaction to access remote databases distributed around the connected information network through the wireless connection. The research on real-time database systems (RTDBS) have received a lot interests in the last decade [15, 17]. Some of the common applications of real-time database technologies are program stock trading, air traffic control systems and battlefield management systems. RTDBS is one whose transactions are associated with time constraints, usually called deadlines [16]. The performance and sometimes the correctness of RTDBS depend on how well the deadlines can be met. Besides meeting transaction deadlines, the RTDBS needs to observe data consistency constraints as well [14]. Since the time constraint and the data consistency constraint impose conflicting demands on system resources, scheduling transactions to meet both constraints thus becomes a tricky business. A number of studies have been done on this subject. For example, the earliest deadline first (EDF) algorithm assigns a higher priority to a transaction with an earlier deadline whereas the High-Priority (HP) concurrency control protocol allows a higher priority transaction to restart a lower priority one in case of data access conflict. Since many applications of RTDBS are distributed in nature, the research on distributed real-time database systems (DRTDBS) also receives growing interests in recent years. The situation becomes more complicate when a database is partitioned into a number of smaller local databases residing at different sites in case of a DRTDBS. Additional overheads are required for the management of distributed transactions, the resolution of ...
Research in the concurrency control of real-time data access over mobile networks is receiving growing attention. With possibly lengthy transmission delay and frequent disconnection, traditional concurrency control mechanisms may become very costly and time-consuming in mobile distributed real-time database systems (MDRTDBS). Due to limited bandwidth and unpredictable behavior of mobile networks, the past research on concurrency control for distributed real-time database systems (DRTDBS) can not be directly applied to MDRTDBS. In this paper, we propose a distributed real-time locking protocol, called SDHP-2PL, based on the High Priority Two Phase Locking (HP-2PL) scheme and the concept of similarity for MDRTDBS. We consider the characteristics of mobile networks and adopt the concept of similarity to reduce the possibility of lock conflicts. A detailed model of a MDRTDBS has been developed, and a series of simulation experiments have been conducted to evaluate the capability of SDHP-2PL and the effectiveness of using similarity for MDRTDBS.The simulation results have confirmed our belief that the use of similarity as the correctness criterion for concurrency control in MDRTDBS can significantly improve the system performance.
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