Distributed Real-time Database Systems Research Articles

Conflicts Resolution in Mobile Distributed Real-Time Database Systems: Issues and Challenges

Conflicts Resolution in Mobile Distributed Real-Time Database Systems: Issues and Challenges

A contention aware EQS priority assignment heuristic for cohorts in DRTDBS

The equal slack (EQS) priority assignment method has shown promising results with distributed real-time systems. As a result, the EQS is considered the first choice for distributed real-time database systems (DRTDBS). However, the integration of EQS with the DRTDBS comes at cost of many issues—intensive data contention due to dynamic cohort priority assignment, global deadlock, and wastage of resources due to cyclic restart. To address the above problems, this paper proposes a contention aware equal slack (CA-EQS) priority assignment heuristic. The CA-EQS heuristic reduces the data contention by utilizing the size of the cohort dependency. After the execution of the first cohort, the information about its size of dependency will be piggybacked with the message that initiates the execution of immediately next cohort of a parent transaction at some other site. The reason for going with the piggybacking approach is that the dependency size of the currently executing cohort not only depends on the data items locked by it but also on the data items that are locked by the cohorts that have already completed their execution before it. This way, a true data contention level can be assessed without any extra overhead (message and time). The CA-EQS solves the problem of deadlock and cyclic restart. Results from the simulation tool validate up to 9% drop in transactions miss ratio and up to 16% reduction in a number of transactions’ rollbacks by the CA-EQS heuristic over EQS, EQF, number of locks (NL), and most dependent transactions first (MDTF).

Transaction Scheduling Heuristics in Mobile Distributed Real Time Database System

Background: Today's widely used small and portable mobile database technologies have geared toward the mobile distributed real-time database systems (MDRTDBS).Various real time applications like mobile devices, missile systems, navigation control systems, satellites and many others are some examples of MDRTDBS. In the new era of technology, a large domain of applications are based on MDRTDBS, meanwhile different intrinsic limitation like disconnection and mobility typically effect on its correct execution. Mobile distributed real-time systems have different wireless constrained such like energy, processing capacity, memory storage facilities and variable network communication channels. In last few years, different applications run on different mobile nodes needed a suitable transaction mechanism to complete their service without failing its deadline. In recent years researchers focused on MDRTDBS, to develop a suitable concurrency control, commit control method. Replication, check pointing, security, caching and query processing are some other hot research topics in the field of MDRTDBS. Objective: it is needed to maintain data consistency and correct results in mobile distributed real time database system. In our review we have identified key issues which might be considered for development of various transaction executing protocols. We have Introduce a taxonomy of different CC, commit, replication and security issues, which could be advantageous for design, and development of transaction protocols. Method: In the review we have discussed various concurrency, commit, replication methods. Apart from these we have discussed various check pointing, caching and query techniques which is developed in database system. A comparison among various concurrency and commit protocols has been done in the review. The role of different key methods which can affect and help the transaction execution in wireless environment is discussed separately in the paper. Results: Analytical results are not mentioned in the review paper. However the role and affect on the transaction execution are mentioned clearly. Issues and their advantages of different concurrency and commit protocols are mentioned. Results: Analytical results are not mentioned in the review paper. However the role and affect on the transaction execution are mentioned clearly. Issues and their advantages of different concurrency and commit protocols are mentioned. Conclusion: It is found that transaction processing is still a challenging area of research. A number of issues has been discussed and reviewed various approaches to control concurrency control and atomicity methods. We have presented a detailed survey and classification of various issues based on commit, concurrency, and replication methods for MTDRTDBS. However, in the paper different security, caching and query processing and check pointing issues has been also discussed which should be considered for future work. Database researchers have needed to integrate these issues with their work and develop a suitable protocol.

IDRC: A Distributed Real-Time Commit Protocol

With rapidly increasing interest in distributed real-time database systems (DRTDBS), data conflicts amongst distributed transactions have become a crucial factor which negatively influences the system performance. To address above issue, anImproved Data Lending based Distributed Real-Time Commit (IDRC) protocol has been developed. IDRC reduce the negative impact of data conflicts on the DRTDBSperformance by increasing the data accessibility as compared to the earlier protocols. It allows the commit dependent borrower participant node (who has already lent the data item to the abort dependent borrower participant node) to lend the same data item in read mode to more than one borrower participant nodes. The simulation results show that the proposed commit protocol outperforms.

Issues Related to Mobile Database System

As the popularity of electronic commerce and maturity of wireless technology increase continuously through which mobile users can access the internet anytime anywhere without having to physically hook up any access point. However protocol, physical constraints, security and cost may still limits communication - it will not be possible to establish online communications for as long as they want, whenever they want. So there is a strong need for developing mobile database system for mobile devices. In this way mobile database opens up a new chapter and provided solutions to this restrictions. There is also support of the real time database system depending upon the timing constraints pulls the mobile database concepts which emerges them in a new form of technology as mobile distributed real time database systems. This paper sheds light on mobile database systems and issues related to MDRTDB.

A new admission control policy and over load detection in DRTDBS

Today's Real Time System (RTS) is characterized by managing large volume of dispersed data making real-time distributed data processing a reality [1,2,14]. However, such a rapid growth of distributed real time database application executes in unpredictable environment where workload varies significantly at run time. Hence, computation workload on a large scale distributed system can lead to numbers of transaction deadline misses [3,4]. Therefore, efficient database management algorithm and protocol for accessing and manipulating data are required to satisfy timing constraints for supported application. In this paper, an algorithm has been proposed for admission control policy and overload detection for Distributed Real Time Database Systems (DRTDBS) consisting of local controller and global load balancer working at each site, which decide whether to admit or reject the newly arrived transaction. The local controller controls the admission process of incoming transaction whereas the global load balancer collects the performance data from other nodes and balances the system wide workload. An outright rejection of a transaction may be safer than attempting to execute that transaction only to miss its deadline. The simulation results show that the new algorithm successfully balances the workload in a distributed real time database and provide lesser transaction miss ratio under tight transaction workload as compared to existing protocols [5-7].

SPEEDITY-A Real Time Commit Protocol

This paper presents Shadow, Piggy bag, Elemental External Dependency Inversion and in Time Yielding (SPEEDITY) commit protocol for distributed real time database systems (DRTDBS). Here, only abort dependent cohort having deadline greater than a specific value (Tshadow_creation_time) needs to forks off a replica of itself called a shadow, whenever it borrows dirty value of a data item. Commit-on-Termination external dependency between final commit of lender and shadow of its borrower and Begin-on-Abort internal dependency between shadow of borrower and borrower itself are defined. Due to heavy delay in commitment of lender in the case of update-read conflict, execution of borrower is started with its shadow by sending YES-VOTE message piggy bagged with the before value [11] to its coordinator after aborting it and abort dependency created between lender and borrower is reversed to commit dependency between shadow and lender with read-update conflict and commit operation governed by Commit-onTermination dependency. The performance of SPEEDITY is compared with shadow PROMPT, SWIFT and DSS-SWIFT commit protocols [6, 22, 23] for both main memory resident and disk resident databases with and without communication delay. Simulation results show that the proposed protocol improves the system performance up to 5% as transaction miss percentage.

ACTIVE-A Real Time Commit Protocol

Many existing real time commit protocols try to improve system performance by allowing a committing cohort to lend its data to an executing cohort, thus reducing data inaccessibility. They block the borrower from sending WORKDONE/PREPARED message and restrict them from lending data so that transaction abort chain is limited to one. Thus, transaction execution time increases. This paper proposes a modified real time commit protocol for distributed real time database systems (DRTDBS), Allow Commit Dependent and in Time borrowers for Incredible Value added data lending without extended abort chain (ACTIVE), where borrower cohorts are categorized as commit and abort dependent. Further, the commit dependent borrowers can lend data to executing cohorts with still limiting the transaction abort chain to one only and reducing the data inaccessibility. Also, an incoming executing cohort having borrowing factor greater than one can only borrow the dirty data items from lender. This minimizes the fruitless borrowing by the cohort. The performance of ACTIVE is compared with PROMPT, 2SC and SWIFT protocols for both main memory resident and disk resident databases with and without communication delay. Simulation results show that the proposed protocol improves the system performance up to 4% as transaction miss percentage.

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.

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.

Priority assignment in distributed real-time databases using optimistic concurrency control

In the studies of distributed real-time database systems (DRTDBS), it is always assumed that earliest deadline first (EDF) is employed as the CPU scheduling algorithm. However, using purely (ultimate) deadline for priority assignment may not be suitable because different kinds of transactions, such as global and local transactions, may exist in the system. To improve the performance, more sophisticated priority assignment heuristics have to be employed. In the paper, the performance of different priority assignment heuristics for subtransactions in DRTDBS using optimistic concurrency control (OCC) protocol are investigated. It is found that purely deadline-driven heuristics, which suffice for distributed real-time systems, are not suitable for DRTDBS. On the other hand, the proposed heuristic, which considers both deadline constraint and data contention, can give the best performance.

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.

The Impact of Network Layer on the Deadline Assignment Strategies in Distributed Real-Time Database Systems

In recent years, Distributed Real-Time Database Systems (DRTDBS) have become one of the most exciting topics in the research of real-time computing systems. In the literature, many features of these systems have been addressed using different simulation models. However, these models always put less emphasis on the network issues. In turn, the networks are usually modeled as simple constant delay servers. However, it is found that the performance of the underlying network has a significant impact on the overall performance of the DRTDBS. The results also showed that the unpredictable behavior of the underlying network will give non-constant network transit time which in turn deter the overall system performance. To alleviate this impact, one of the methods is to assign virtual deadlines to those affected transactions such that preferences or higher priorities can be given in the scheduling of resources. In this paper, the impacts of different network issues are addressed. Then, the use of deadline assignment in transaction scheduling is discussed. Finally, a virtual deadline assignment scheme is proposed. The scheme assigns a virtual deadline based on the current network state to those transactions which require a comparatively better performance. Our results demonstrate that dynamically tuning the virtual deadlines of transactions according to the state of the network, yields a substantial improvement in the performance of that certain type of transactions without severely affecting the overall system performance

Concurrency Control for Time-constrained Transactions in Distributed Databases Systems

The design of concurrency control protocols for time-constrained transactions is complicated due to the requirements to maintain the database consistency and to satisfy the timing constraints of the transactions. In the past few years, various real-time locking protocols have been proposed for different real-time database systems (RTDBS). However, the use of these protocols for distributed real-time database systems (DRTDBS) has received much less attention, even though many RTDBS are distributed in nature. In this paper, two efficient real-time locking protocols are proposed for DRTDBS. The first one, based on dynamic locking, is called Distributed Hybrid Two Phase Locking (DHb2PL). Its performance has been compared in detail with three other distributed real-time locking protocols. The performance results indicate that DHb2PL is much better than the other protocols as a result of a better approach to resolving lock conflicts and its deadlock free property. The second one, called DRT-S2PL, is based on static locking where the locks required by a transaction are assumed to be known before its execution. Its relative performance as compared with DHb2PL is dependent on the proportion of remote locks required by a transaction. DRT-S2PL is more suitable for systems with transactions which have to set a large proportion of remote locks.