Kam-Yiu Lam

Cache invalidation scheme for mobile computing systems with real-time data

In this paper, we propose a cache invalidation scheme called Invalidation by Absolute Validity Interval (IAVI) for mobile computing systems. In IAVI, we define an absolute validity interval (AVI), for each data item based on its dynamic property such as the update interval. A mobile client can verify the validity of a cached item by comparing the last update time and its AVI. A cached item is invalidated if the current time is greater than the last update time plus its AVI. With this self-invalidation mechanism, the IAVI scheme uses the invalidation report to inform the mobile clients about changes in AVIs rather than the update event of the data items. As a result, the size of the invalidation report can be reduced significantly. Through extensive simulation experiments, we have found that the performance of the IVAI scheme is significantly better than other methods such as bit sequence and timestamp.

An efficient method for generating location updates for processing of location-dependent continuous queries

Recent advances in mobile computing and mobile communication technology have led to the emergence of many innovative mobile computing applications. Some of them require providing support to location-dependent continuous queries (LDCQs) on moving objects. The result of a location-dependent query depends on the current locations of the moving objects. When the query is specified as continuous, the requesting client can get continuously changing results. In order to provide correct and timely results to requesting clients, the locations of moving objects have to be closely monitored. In this paper, we propose an adaptive monitoring method (AMM) for managing the locations of moving objects to maintain the correctness of the results of query evaluation without significantly increasing the wireless bandwidth requirements. Extensive simulation experiments have been conducted to investigate the performance of the proposed method as compared to plain dead-reckoning (PDR).

Concurrency control in mobile distributed real-time database systems

Abstract With the rapid advances in mobile computing technology, there is an increasing demand for processing real-time transactions in a mobile environment. This paper studies concurrency control problem in mobile distributed real-time database systems (MDRTDBS). Based on the High Priority Two Phase Locking (HP-2PL) scheme, we propose a distributed real-time locking protocol, called Distributed High Priority Two Phase Locking (DHP-2PL), for MDRTDBS. In the protocol, the characteristics of a mobile computing system are considered in resolving lock conflicts. Two strategies are proposed to further improve the system performance and to reduce the impact of mobile network on the performance of the DHP-2PL: (1) A transaction shipping approach is proposed to process transactions in a mobile environment by exploring the well-defined behavior of real-time transactions. (2) We explore the application semantics of real-time database applications by adopting the notion of similarity in concurrency control to further reduce the number of transaction restarts due to priority inversion, which could be very costly in a mobile network. A detailed simulation model of a MDRTDBS has been developed, and a series of simulation experiments have been conducted to evaluate the performance of the proposed approaches and the effectiveness of using similarity for concurrency control in MDRTDBS.

Deferrable Scheduling for Maintaining Real-Time Data Freshness: Algorithms, Analysis, and Results

The periodic update transaction model has been used to maintain the freshness (or temporal validity) of real-time data. Period and deadline assignment has been the main focus of past studies, such as the More-Less scheme [25], in which update transactions are guaranteed by the Deadline Monotonic scheduling algorithm [16] to complete by their deadlines. In this paper, we propose a deferrable scheduling algorithm for fixed-priority transactions, a novel approach for minimizing update workload while maintaining the temporal validity of real-time data. In contrast to prior work on maintaining data freshness periodically, update transactions follow an aperiodic task model in the deferrable scheduling algorithm. The deferrable scheduling algorithm exploits the semantics of temporal validity constraint of real-time data by judiciously deferring the sampling times of update transaction jobs as late as possible. We present a theoretical estimation of its processor utilization and a sufficient condition for its schedulability. Our experimental results verify the theoretical estimation of the processor utilization. We demonstrate through the experiments that the deferrable scheduling algorithm is an effective approach and it significantly outperforms the More-Less scheme in terms of reducing processor workload.

A deferrable scheduling algorithm for real-time transactions maintaining data freshness

Periodic update transaction model has been used to maintain freshness (or temporal validity) of real-time data. Period and deadline assignment has been the main focus in the past studies such as the more-less scheme by Xiong and Ramamrithan (2004) in which update transactions are guaranteed by the deadline monotonic scheduling algorithm by Leung and Whitehead (1982) to complete by their deadlines. In this paper, we propose a novel algorithm, namely deferrable scheduling, for minimizing imposed workload while maintaining temporal validity of real-time data. In contrast to previous work, update transactions scheduled by the deferrable scheduling algorithm follow a sporadic task model. The deferrable scheduling algorithm exploits the semantics of temporal validity constraint of real-time data by judiciously deferring the sampling times of update transaction jobs as late as possible. We present a theoretical analysis of its processor utilization, which is verified in our experiments. Our experimental results also demonstrate that the deferrable scheduling algorithm is a very effective approach, and it significantly outperforms the more-less scheme in terms of reducing processor workload

Maintaining temporal consistency of discrete objects in soft real-time database systems

A real-time database system contains base data items which record and model a physical, real-world environment. For better decision support, base data items are summarized and correlated to derive ...A real-time database system contains base data items which record and model a physical, real-world environment. For better decision support, base data items are summarized and correlated to derive views. These base data and views are accessed by application transactions to generate the ultimate actions taken by the system. As the environment changes, updates are applied to base data, which subsequently trigger view recomputations. There are thus three types of activities: base data update, view recomputation, and transaction execution. In a real-time database system, two timing constraints need to be enforced. We require that transactions meet their deadlines (transaction timeliness) and read fresh data (data timeliness). In this paper, we define the concept of absolute and relative temporal consistency from the perspective of transactions for discrete data objects. We address the important issue of transaction scheduling among the three types of activities such that the two timing requirements can be met. We also discuss how a real-time database system should be designed to enforce different levels of temporal consistency.

Priority Scheduling of Transactions in Distributed Real-TimeDatabases

One of the most important issues in the design of distributed real-time database system (DRTDBS) is transaction scheduling which consists of two parts: priority scheduling and real-time concurrency control. In the past studies, mostly, these issues are studied separately although they have a very close interaction with each other. In this paper, we propose new priority assignment policies for DRTDBS and study their impact on two typical real-time concurrency control protocols (RT-CCPs), High Priority Two Phase Locking (HP-2PL) and Optimistic Concurrency Control with Broadcast Commit (OCC-BC). Our performance results show that many factors, such as data conflict resolution, degree of data contention and transaction restarts, that are unique to database systems, have significant impact on the performance of the policies which in turn affect the performance of the real-time concurrency control protocols. OCC-BC is more affected by the priority assignment policies than HP-2PL owing to the late detection of conflict. In the design of priority assignment policies, we have found that neither the purely deadline driven policies nor data contention driven policies are suitable for DRTDBS. Our proposed policy, the Mixed Method (MM), which considers both transaction timeliness and data contention, outperforms other policies over a wide range of system parameter settings.

Resolving Executing-Committing Conflicts in Distributed Real-time Database Systems

In a distributed real-time database system (DRTDBS), a commit protocol is required to ensure transaction failure atomicity. If data conflicts occur between executing and committing transactions, the performance of the system may be greatly affected. In this paper, we propose a new protocol, called deadline-driven conflict resolution (DDCR), which integrates concurrency control and transaction commitment management for resolving executing and committing data conflicts amongst firm real-time transactions. With the DDCR, a higher degree of concurrency can be achieved, as many data conflicts of such kind can be alleviated, and executing transactions can access data items which are being held by committing transactions in conflicting modes. Also, the impact of temporary failures which occurred during the commitment of a transaction on other transactions, and the dependencies created due to sharing of data items is much reduced by reversing the dependencies between the transactions. A simulation model has been developed and extensive simulation experiments have been performed to compare the performance of the DDCR with other protocols such as the Opt [1], the Healthy-Opt [2], and the base protocol, which use priority inheritance and blocking to resolve the data conflicts. The simulation results show that the DDCR can significantly improve the system performance under different workload and workload distributions. Its performance is consistently better than the base protocol and the Opt protocols in both main-memory resident and disk-resident DRTDBS.

On Using Real-Time Static Locking Protocols for DistributedReal-Time Databases

The use of Static Two Phase Locking Protocols (S2PL) for concurrency control in real-time database systems (RTDBS) has received little attention in the past. Actually, real-time S2PL (RT-S2PL) protocols do possess some desirable features making them suitable for RTDBS, especially for distributed real-time database systems (DRTDBS) in which remote locking is required and distributed deadlock is possible. In this paper, different RT-S2PL protocols are proposed. They differ in their methods of reducing the blocking time of higher priority transactions. Their performance is studied and compared with a real-time dynamic two phase locking protocol (RT-D2PL), called Hybrid Two Phase Locking (Hb2PL). The impact of different system and workload parameters, such as mean inter-arrival time of transactions, number of remote lock requests of a transaction, communication overhead for sending messages, and database size on their performance have been examined. The performance results indicate that the RT-S2PL protocols are suitable for DRTDBS in which the proportion of local locks of a transaction is small and the communication overhead for locking is high.

Real-time optimistic concurrency control protocol with dynamic adjustment of serialization order

Proposes a new real-time optimistic protocol. By using a dynamic adjustment of the serialization order by backward-adjusting the non-serious conflicting transactions before the committing transactions, many unnecessary restarts can be eliminated. In the protocol, no conflict or serialization constraints have to be recorded during the read phase of a transaction. Different priority conflict resolution methods can be easily incorporated in the protocol. In addition, with the use of Thomass write rule, the number of transaction restarts can further be much reduced.