In this paper, we study the performance and impact of maintaining temporal consistency on a recently proposed concurrency control protocol for processing transactions in broadcast environments. This protocol offers autonomy between mobile clients and the server such that mobile clients can read consistent data off the air without contacting the server. However, most of the existing mobile computing applications, such as information dispersal systems for stock prices and weather information, are comprised of real-time read only transactions. In order to deliver timely and useful results, real-time transactions must also read temporal consistent data. A number of approaches to maintaining temporal consistency are studied through a series of simulation experiments. Results show that taking advantage of data semantics and temporal consistency requirement can improve the performance of mobile read only transactions in broadcast environments.

Many studies on transaction processing in mobile environments have an implicit assumption that the server is able to broadcast consistent data to the mobile clients. However, this assumption may not be valid unless there is a special algorithm to handle broadcasting data in a consistent and timely manner. In this paper, we formulate broadcasting data at the server as a broadcast transaction which reads the entire database in a consistent way in the broadcasting process. This issue is not trivial as the broadcast transaction will create high interference to normal transactions at the server.

Most of the recent studies on broadcast based data dissemination approach have an implicit assumption that the server is able to broadcast consistent data to the mobile clients. However, this assumption may not be valid unless there is a special algorithm to handle broadcasting data in a consistent and timely manner. In this work, we address a number of issues related to this problem. An efficient algorithm called the read-write set test is adopted to read the entire database, which causes little interference with the update transactions at the server. For broadcast transaction scheduling, an algorithm based on a mixed strategy is proposed to handle the case of flat broadcast disks. Using these two algorithms together with some fine tuning, the objectives of reading entire database consistently with minimum impact to the system, broadcasting fresh data, and meeting the periodicity requirement can be met.

In the near future, tens of millions of users will have access to distributed information systems through the wireless interface in their mobile computers. The physical characteristics of the wireless medium pose new challenging issues on data consistency of transaction processing in wireless environments. In particular, the monetarily expensive wireless bandwidth and the asymmetric communication between the mobile clients and the server bar the applicability of conventional transaction processing techniques. In this paper, we first propose a concurrency control protocol in wireless environments that is adapted from the optimistic concurrency control with forward validation protocol. The protocol offers autonomy between the mobile clients and the server such that the mobile clients can read consistent data off the air without contacting the server. To reduce the number of unnecessary transaction restarts such that the timeliness of mobile transactions can be enhanced, we propose another protocol based on timestamp ordering that can show significant performance improvement. The timestamp ordering technique enjoys a number of benefits from the separate processing and flexible adjustment of serialization order by exploiting the semantics of read-only transactions. Most of the existing applications, such as information dispersal systems for stock prices, weather information and traffic condition updates, are comprised of read-only transactions. The simulation results confirm that the proposed protocol could be an efficient and effective approach to transaction processing in real-time broadcast environments for meeting transaction deadlines.

Many recent studies have suggested that the optimistic concurrency control (OCC) protocols outperform the locking-based protocols in real-time database systems (RTDBS). However, the OCC protocols suffer from the problem of unnecessary transaction restarts that is detrimental to transactions meeting their deadlines. The problem is more intensified in mixed transaction environments. Firm transactions are more vulnerable to restarts when they are in conflict with hard transactions on data access. In this paper, we addressed the problem and devised an effective OCC protocol with dynamic adjustment of serialization order, called OCC-DA, for RTDBS with mixed transactions. This protocol can avoid unnecessary transaction restarts by dynamically adjusting the serialization order of the conflicting transactions with respect to the validating transaction. As a result, much resource can be saved and more firm transactions can meet their deadlines without affecting the execution of hard transactions. The characteristics of the OCC-DA protocol were examined in detail by simulation. The results show that the performance of the OCC-DA protocol was consistently better than the other two popular protocols, OCC with forward validation and OCC with Wait-50, over a wide range of system settings. In particular, the OCC-DA protocol provides a more significant performance gain in mixed transaction environments.

Conventional concurrency control protocols are inapplicable in mobile broadcast environments due to a number of constraints of wireless communications. Previous studies are focused on efficient processing of read-only transactions at the mobile clients, neglecting update transactions. In this paper, we design a new protocol for processing both read-only and update mobile transactions. The protocol can detect data conflicts at an early stage at the mobile clients and resolve data conflicts flexibly using dynamic adjustment of timestamp ordering. Early data conflict detection saves processing and communication resources, while dynamic adjustment of timestamp ordering allows more schedules of transaction executions such that unnecessary transaction aborts can be avoided. We performed extensive simulation studies to evaluate the effectiveness of these two features for the performance of the new protocol. The analysis of simulation results showed that both features are effective and contribute differently to the satisfactory performance of the protocol.

In broadcast environments, communication bandwidth asymmetry is a distinguishing feature. The low bandwidth available for clients to communicate with the broadcast server makes it expensive and inefficient to maintain global serializability among all the transactions that execute at distributed mobile clients. In this paper, investigation is made to relax the requirements of global serializability among transactions while data consistency is still achieved. The NRS-Algorithm is proposed to process read-only transactions separately from update transactions to reduce the degree of data contention between the two kinds of transactions. Moreover, each read-only transaction running at the clients is guaranteed to read consistent data values autonomously without the need to do validation at the server. This is extremely beneficial to the concurrency control in broadcast environments where the upstream communication capacity is limited. To generalize the idea, two kinds of group consistency requirements for read-only transactions are defined. Group strong consistency requires a group of read-only transactions to be conflict serializable with respect to the update transactions. A weaker form of consistency, group view consistency, is also defined to allow a group of read-only transactions to be view serializable with respect to some update transactions. The NRS-Algorithm is then extended to preserve the two kinds of group consistency within a group of read-only transactions.

In this paper, we investigate the approach of using separate algorithms to process read-only transactions in real-time systems. A read-only transaction (ROT) is a transaction that only reads, but does not update any data item. Since there is a significant proportion of ROTs in several real-time systems, it is important to investigate how to process ROTs effectively. Using an algorithm to process ROTs separately from update transactions may reduce the interference between ROTs and update transactions. This reduced interference alleviates the impact of concurrency control on real-time priority-driven scheduling and improves the timeliness of the system. Moreover, we explore the different consistency requirements of ROTs. Particularly, we define a weaker form of consistency, view consistency, which allows ROTs to perceive different serialization order of update transactions. While view consistency permits non-serializability, ROTs are still ensured to see consistent data. We propose two robust algorithms for different consistency requirements of ROTs. The two algorithms are robust in the sense that they can be used in a compatible way so that a real-time system can provide different consistent data for different applications. The performance of two algorithms was examined through a series of simulation studies. The simulation results show that the two algorithms outperform the high-priority two-phase locking protocol.

The difficulties of providing a guarantee of meeting transaction deadlines in hard real-time database systems lie in the problems of priority inversion and of deadlocks. Priority inversion and deadlock problems ensue when concurrency control protocols are adapted in priority-driven scheduling. The blocking delay due to priority inversion can be unbounded, which is unacceptable in the mission-critical real-time applications. Some priority ceiling protocols have been proposed to tackle these two problems. However, they are too conservative in scheduling transactions for the single-blocking and deadlock-free properties, leading to many unnecessary transaction blockings. The authors analyze the unnecessary transaction blocking problem inherent in these priority ceiling protocols and investigate the conditions for allowing a higher priority transaction to preempt a lower priority transaction using the notion of dynamic adjustment of serialization order. A new priority ceiling protocol is proposed to solve the unnecessary blocking problem, thus enhancing schedulability. They also devise the worst-case schedulability analysis for the new protocol which provides a better schedulability condition than other protocols.