Real-time Database Systems Research Articles

Scheduling transactions in mobile distributed real-time database systems

A DMVOCC-MVDA (distributed multiversion optimistic concurrency control with multiversion dynamic adjustment) protocol was presented to process mobile distributed real-time transaction in mobile broadcast environments. At the mobile hosts, all transactions perform local pre-validation. The local pre-validation process is carried out against the committed transactions at the server in the last broadcast cycle. Transactions that survive in local pre-validation must be submitted to the server for local final validation. The new protocol eliminates conflicts between mobile read-only and mobile update transactions, and resolves data conflicts flexibly by using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. Mobile read-only transactions can be committed with no-blocking, and respond time of mobile read-only transactions is greatly shortened. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. In global validation mobile distributed transactions have to do check to ensure distributed serializability in all participants. The simulation results show that the new concurrency control protocol proposed offers better performance than other protocols in terms of miss rate, restart rate, commit rate. Under high work load (think time is 1s) the miss rate of DMVOCC-MVDA is only 14.6%, is significantly lower than that of other protocols. The restart rate of DMVOCC-MVDA is only 32.3%, showing that DMVOCC-MVDA can effectively reduce the restart rate of mobile transactions. And the commit rate of DMVOCC-MVDA is up to 61.2%, which is obviously higher than that of other protocols.

하드와 소프트 실시간 트랜잭션을 위한 통합된 동시성제어 기법

Most concurrency control protocols have been devised for resolving data conflicts among real-time transactions of a single type. Recent real-time database systems should support various types of real-time transactions due to needs of many different types of applications and steady improvement of hardware. In this paper, we propose integrated concurrency control protocol to resolve data conflicts among hard and soft real-time transactions. Our proposed protocol, based on PCP(Priority Ceiling Protocol) for a hard real-time transactions and MVPR(Multiversion with Precedence Relationship), guarantees that hard real-time transactions meet their deadline, and decreases the deadline miss ratio of soft real-time transactions. We also proved that the proposed protocol guarantees serializable schedules and no deadlocks. The performance of the proposed protocol has been compared with other real-time concurrency protocols.

Distributed real time database systems: background and literature review

Today's real-time systems (RTS) are characterized by managing large volumes of dispersed data making real-time distributed data processing a reality. Large business houses need to do distributed processing for many reasons, and they often must do it in order to stay competitive. So, efficient database management algorithms and protocols for accessing and manipulating data are required to satisfy timing constraints of supported applications. Therefore, new research in distributed real-time database systems (DRTDBS) is needed to investigate possible ways of applying database systems technology to real-time systems. This paper first discusses the performance issues that are important to DRTDBS, and then surveys the research that has been done so far on the issues like priority assignment policy, commit protocols and optimizing the use of memory in non-replicated/replicated environment pertaining to distributed real time transaction processing. In fact, this study provides a foundation for addressing performance issues important for the management of very large real time data and pointer to other publications in journals and conference proceedings for further investigation of unanswered research questions.

On new scheduling policy for the improvement of firm RTDBSs performances

Earliest deadline first (EDF) is one of the main scheduling policies used in real-time database systems (RTDBSs) for transactions processing. With EDF, prioritized transactions are not necessarily the most important in the system. Moreover, it is well-known that EDF is not efficient in overload conditions. In this paper, we introduce the notion of transaction importance and present a new priority assignment technique based on both transactions importance and deadlines. This assignment policy leads to a new scheduling policy, called generalized earliest deadline first (GEDF). In order to show the benefits of using GEDF for managing real-time transactions, we have designed an RTDBS simulator and carried out Monte Carlo simulations.

Simulating Concurrency Control with Deadlock Avoidance in Real-Time Transaction Processing

The correctness of real-time database systems depends on both the data consistency and the timeliness of the results. We present a very comprehensive study which demonstrates the effect of using various priority assignment and concurrency control protocols with a variety of underlying system configurations, including use of multiple disks, CPUs, and buffer sizes. The study identifies various bottlenecks and solutions under varying system loads and transaction parameters. Both preemptive and non-preemptive protocols are studied under realistic assumptions. Finally, a performance comparison of such a system operating under deadlock and no-deadlock conditions provides insight into the potential waste of resources when concurrency is restricted.

A dynamic crash recovery scheme for distributed real-time database systems

Recovery performance in the event of failures is very important for distributed real-time database systems. This paper presents a time-cognizant logging-based crash recovery scheme (TCLCRS) that aims at distributed real-time databases, which adopts a main memory database as its ground support, hi our scheme, each site maintains a real-time log for local transactions and the sub-transactions, which execute at the site, and execute local checkpointing independently. Log records are stored in non-volatile high-speed store, which is divided into four different partitions based on transaction classes. During restart recovery after a site crash, partitioned crash recovery strategy is adopted to ensure that the site can be brought up before the entire local secondary database is reloaded in main memory. The partitioned crash recovery strategy not only guarantees the internal consistency to be recovered, but also guarantee the temporal consistency and recovery of the sates of physical world influenced by uncommitted transactions. Combined with two-phase commit protocol, TCLCRS can guarantee failure atomicity of distributed real-time transactions.

On Optimistic Concurrency Control for Real-Time Database Systems

The performance of database transaction processing system can be profoundly affected by the concurrency control method employed since it is necessary to preserve database integrity in a multi-user environment. In addition to satisfying the consistency requirement as in traditional database system, real-time database systems must also satisfy timing constraints. In this study we present a virtual run policy for the restarted first run optimistic transactions and compare its performance with optimistic concurrency control in firm real-time database system.

Architecture of experimental real time database for embedded systems

With the continuous advancement of technology many embedded and real time systems require increasingly larger amount of information to be handled and managed in a timely manner. Thus, an efficient data management mechanism is necessary and that is why there is a noticeable interest in merging standard database technology and real-time technology recently. It is very difficult to achieve guaranteed real-time database services because various components can compete for system resources. Our objective is to design an experimental real-time database system which would be suitable enough to study the most important real-time database issues. Proposed system can help us to understand the cffcct of various database components on system performance and to identify the most important factors

DESIGN OF EXPERIMENTAL PLATFORM FOR TESTING REAL-TIME DATABASE TRANSACTION PROCESSING

It is very difficult to achieve guaranteed real time database services when putting a database into a real-time environment because various components can compete for system resources. Previous research in real-time databases has focused primarily on evolution of transaction processing algorithms, priority assignment strategies and concurrency control techniques. But for the most part the research efforts are based only on simulation studies with many parameters defined. Our objective is to design and implement an experimental real-time database system suitable for study of real time transaction processing. The experimental system is implemented as an integrated set of the most important functional parts upon the real-time operating system VxWorks. It serves as a support platform for performance evaluation of known and new algorithms of the particular processing components to understand their effect on system performance and to identify the most influencing factors.

Utilization of execution histories in scheduling real-time database transactions

Real-time database systems support data processing needs of real-time systems where transactions have time constraints. Here we consider repetitively executed transactions, and assume that execution histories are logged. A well-known priority assignment technique called earliest-deadline-first is biased towards short transactions in which short transactions have better chances of completing their executions within their deadlines. We introduce the notion of “fair scheduling” in which the goal is to have “similar” completion ratios for all transaction classes (short to long in sizes). We propose priority assignment techniques that overcome the biased scheduling and show that they work via extensive simulation experiments.

Real-time Optimistic Concurrency Control based on Transaction Finish Degree

Concurrency control is one of the main issues in the studies of real-time database systems. Optimistic concurrency control algorithms have the attractive properties of being non-blocking and deadlock-free. However, they have the problems of late conflict detection and transaction restarts. Although the number of transaction restarts is reduced by dynamic adjustment of serialization order in real-time database systems, they are still some unnecessary transaction restarts. In this study, we propose a new method called Transaction Finish Degree (TFD) and a new Multiversion Optimistic Concurrency Control algorithm based on TFD (MVOCC-TFD), which can reduce the number of unnecessary restarts. Theoretical analysis and experimental results demonstrate that the new algorithm can outperform the previous ones

Development of Real Time Database System for Disaster Prevention of Small Earth Dams

Development of Real Time Database System for Disaster Prevention of Small Earth Dams

Scheduling Value-Based Nested Transactions in Distributed Real-Time Database Systems

Many noticeable studies have focussed on scheduling flat transactions in a distributed real-time database system (RTDBS). However, a nested transaction model has been widely adopted in many real-life applications such as Internet stock trading systems and telecommunications. This work concerns efficiently scheduling real-time nested transactions in a distributed RTDBS. A new real-time scheduler called flexible high reward for nested transactions (FHRN) is proposed. FHRN consists of (1) FHRNp1 policy to schedule real-time nested transactions and (2) 2PLlHPN to resolve the concurrent data-accessing problem among interleaved nested transactions. Simulation results show that FHRN outperforms these existent real-time schedulers such as random priority (RP), earliest deadline (ED), highest value (HV), hierarchical earliest deadline (HED), and highest reward and urgency (HRU) when an application requires a nested transaction model.

Parallel Concurrency Control Activity for Transaction Management in Real-time Database Systems

In a real-time database system, an application supports a mix of transactions. These include the real-time transactions that require completion by a given deadline. Time-critical requirements also exist in many distributed multi-media system applications. Existing concurrency control procedures introduce excessive delays due to non-availability of data resources. In this study, we ignore the delays incurred by ordinary transactions, in order to achieve a non-interference mode of execution (near parallel) for the time-critical transactions. For this purpose, a data allocation model has been studied. It is a stochastic process model based on the use of two-phase locking. It highlights the available possibilities for reductions of delays for time-critical transactions within a distributed real-time database systems. Based on the new conceptual model, modified synchronization techniques for time-critical transactions have been proposed.

An alarm management framework for automated network fault identification

Many timing constraint (or real-time) distributed systems, such as real-time database systems, are now being used in safety critical applications. However, they are subject to system failures caused by the malfunction of underlying network components. Without the helps of network experts or sophisticated management tools, most users cannot resolve these network problems by themselves. Sometimes, worse, it is usually prohibited to use these management tools, e.g. the ‘ping’ command, for the security sake. Accordingly, we develop a management system to automate network fault identification merely based on the analysis of the abnormal events from the monitored timing constraint distributed system. In this system, a fault identification framework is designed to identify automatically faulty network elements by using a two-level fault propagation model which combines Timing Constraint Petri nets with an alarm clustering mechanism. In addition, the concepts of redundant/ringleader alarms and innocent network elements are also introduced into the framework to obtain an effective diagnosis. At last, the management system is implemented according to the framework to demonstrate the performance of our fault identification.

An alarm management framework for automated network fault identification

Many timing constraint (or real-time) distributed systems, such as real-time database systems, are now being used in safety critical applications. However, they are subject to system failures caused by the malfunction of underlying network components. Without the helps of network experts or sophisticated management tools, most users cannot resolve these network problems by themselves. Sometimes, worse, it is usually prohibited to use these management tools, e.g. the ‘ping’ command, for the security sake. Accordingly, we develop a management system to automate network fault identification merely based on the analysis of the abnormal events from the monitored timing constraint distributed system. In this system, a fault identification framework is designed to identify automatically faulty network elements by using a two-level fault propagation model which combines Timing Constraint Petri nets with an alarm clustering mechanism. In addition, the concepts of redundant/ringleader alarms and innocent network elements are also introduced into the framework to obtain an effective diagnosis. At last, the management system is implemented according to the framework to demonstrate the performance of our fault identification.

A commit strategy for distributed real-time transaction

Ramamritham gives three common types of constraints for the execution history of concurrent transactions. This paper extends the constraints and gives the fourth type of constraint. Then the weak commit dependency and abort dependency between transactions, because of data access conflicts, are analyzed. Based on the analysis, an optimistic commit protocol 2LC (two-Level Commit) is proposed, which is specially designed for the distributed real-time domain. It allows transactions to optimistically access the locked data in a controlled manner, which reduces the data inaccessibility and priority inversion inherent and undesirable in distributed real-time database systems. Furthermore, if the prepared transaction is aborted, the transactions in its weak commit dependency set will execute as normal according to 2LC. Extensive simulation experiments have been performed to compare the performance of 2LC with that of the base protocol, the permits reading of modified prepared-data for timeliness (PROMPT) and the deadline-driven conflict resolution (DDCR). The simulation results show that 2LC is effective in reducing the number of missed transaction deadlines. Furthermore, it is easy to be incorporated with the existing concurrency control protocols.

High performance distributed real-time commit protocol

In a distributed real-time database system, the only way to ensure transaction atomicity is to investigate and develop a real-time atomic commit protocol. This paper presents the model of distributed real-time transaction and analyses all kinds of dependencies because of data conflicts access. Based on this model, we propose an optimistic real-time commit protocol, double space commit (2SC), which is specifically designed for the high-performance distributed real-time transaction. 2SC allows a non-healthy transaction to lend its held data to the transactions in its commit dependency set. When the prepared transaction aborts, only the transactions in its abort dependency set are aborted while the transactions in its commit dependency set will execute as normal. The two properties of 2SC can reduce the data inaccessibility and the priority inversion that is inherent in distributed real-time commit processing. Extensive simulation experiments have been performed to compare the performance of the 2SC with that of other protocols such as the base protocol, the permits reading of modified prepared-data for timeliness [IEEE Transactions on Parallel and Distributed Systems 11 (2) (2000) 160–181] and the deadline-driven conflict resolution [The Computer Journal 42 (8) (1999) 674–692]. The simulation results show that 2SC has the best performance. Furthermore, it is easy to incorporate in current concurrency control protocols.

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 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.

An adaptive scheduler for distributed real-time database systems

In a distributed real-time database system, a remote transaction usually accesses some data located at remote sites. However, traditional real-time schedulers do not consider the impact of communication delay in transferring remote data, resulting in a high MissRatio for remote transactions. This study proposes a new real-time scheduler called flexible high reward (FHR) to reduce the MissRatio and lessen the unnecessary waiting time due to communication delay for a remote transaction. This scheduler gives a remote transaction a slightly higher priority and adjusts a remote transaction’s waiting status such that the resources held by a waiting remote transaction can be released as soon as possible. Through the underlying operating system, local transactions can also be executed while a remote transaction is waiting for remote data. Simulation results demonstrate that the FHR outperforms current real-time schedulers such as earliest deadline, highest value, and highest reward and urgency.