Dynamic Adjustment Of Serialization Order Research Articles

A Multiversion Optimistic Concurrency Control Protocol in Mobile Broadcast Environments

This paper presents MVOCC-MDA-2PV (multiversion optimistic concurrency control with multiversion dynamic adjustment using two-phase validation) protocol for processing mobile real-time transaction in mobile broadcast environments. Transaction validation is performed at two phases: pre-validation and final validation. At the mobile herts (MHs), all mobile transactions (MT) perform pre-validation of transactions by using backward validation mechanism. The pre-validation process is carried out against committed transactions at the server in the last broadcast cycle. Such an early data conflict detection feature can save processing and communication resources. Transactions that survive in pre-validation must be submitted to the server for final validation. The MHs do not need to synchronize with each other and servers for data conflict detection. The protocol can eliminate conflicts between mobile read-only and mobile update transactions, and resolve data conflicts flexibly using multiversion dynamic adjustment of serialization order to avoid unnecessary restarts of transactions. For a mobile ready-only transaction, it can be committed locally in the MH if it passes all the pre-validation in the course of it execution. Respond time of mobile read-only transactions (MROT) is greatly improved. The results of experiment show that the new protocol proposed offers better performance in terms of miss rate and commit rate.

Concurrency control in mobile distributed real-time database systems

This paper presents DMVOCC-2PLV (Distributed Multiversion Optimistic Concurrency Control with Two-Phase Local Validation) protocol for processing mobile distributed real-time transactions in mobile broadcast environments. In DMVOCC-2PLV protocol, transaction validation is performed at two levels: local validation and global validation. Local validation of transactions is performed in two phases: local pre-validation and local final validation. At the MHs (Mobile Hosts), all mobile transactions perform local pre-validation of transactions by using a backward validation mechanism. The local pre-validation process is carried out against committed transactions at the server in the last broadcast cycle. Such an early data conflict detection feature can save processing and communication resources. Transactions that survive in local pre-validation must be submitted to the server for local final validation. In global validation distributed update transactions have to check to ensure distributed serializability in all participants. The protocol can eliminate conflicts between mobile read-only and mobile update transactions, and resolve data conflicts flexibly 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 reduced. The tolerance of mobile transactions of disconnections from the broadcast channel is increased. The results of simulation experiment show that the new protocol proposed offers better performance in terms of miss rate, restart rate, commit rate and throughput.

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.

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

Concurrency control for mixed transactions in real-time databases

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.

Scheduling transactions with stringent real-time constraints

Many real-time database systems (RTDBS) are used to support mission-critical applications such as avionics control systems and defense systems, in which transactions have stringent real-time constraints. In these hard real-time systems, a transaction missing its deadline could lead to catastrophic consequences. The difficulties of providing a guarantee of meeting transaction deadlines in hard RTDBS lie in the problems of priority inversion and deadlocks. The transaction blocking delay due to priority inversion can be unbounded, which is unacceptable in most hard RTDBS. Several priority ceiling protocols have been proposed to solve 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 main reason for their conservatism is their implicit assumption that if a transaction's operation conflicts with that of another executing transaction, it is presumably unable to preempt the conflicting transaction. In this paper, we propose a new priority ceiling protocol that allows a higher priority transaction to preempt a lower priority transaction on data conflicts by using the notion of dynamic adjustment of serialization order. We devise a worst-case schedulability analysis for the new protocol which provides a better schedulability condition compared to other protocols. The simulation results show that the new priority ceiling protocol outperforms other protocols.

Transaction Scheduling in Distributed Real-Time Systems

In this paper, we study the performance of using optimistic approach to concurrency control in distributed real-time database systems (RTDBS). The traditional optimistic approach suffers from the problem of unnecessary restarts. Transaction restarts can significantly increase the system workload and intensify resource and data contention. In distributed environments, the complexity of the system and the high communication overhead exacerbate the problem. Therefore, the number of unnecessary restarts is the determinant factor that affects the performance of optimistic approach in distributed RTDBS. When optimistic approach is extended to distributed environments, a number of issues resulting from the increased complexity and communication overhead have to be resolved. In this paper, a new real-time distributed optimistic concurrency control (DOCC) protocol with dynamic adjustment of serialization order (DASO), called DOCC-DA is proposed. This protocol can avoid unnecessary transaction restarts by dynamically adjusting the serialization order of the conflicting transactions. Therefore, resources can be saved and more transactions can meet their deadlines. In the DOCC-DA protocol, a new distributed circular validation scheme is included to facilitate transaction validation in distributed environments. The performance of the DOCC-DA protocol has been examined in detail by simulation. The results showed that the performance of the DOCC-DA protocol is consistently better than that of other protocols.

Optimistic concurrency control protocol for real-time databases

In recent years, the use of optimistic schemes for concurrency control in real-time database systems (RTDBS) has received more and more attention. Different real-time optimistic protocols have been proposed. They incorporate different priority conflict resolution methods into the validation phase of a transaction to give preferences for the commitment of higher priority transactions. However, some of these protocols suffer from the problem of heavy restart overheads. Restarting near-to-complete transactions greatly increases the system workload and the deadline missing probability of the transactions (Haritsa et al., 1992; Huang, 1991). In order to reduce the number of restarts, dynamic adjustment of the serialization order of the conflicting transactions has been suggested using timestamp intervals to record the transactions' serialization constraints (Lee and Son, 1993a, 1993b; Lee, 1994). However, these kinds of protocols have the problem of heavy implementation overheads as the timestamp intervals have to be checked and updated in case of any conflict even though the transactions are still in their read phases (Lee, 1994). In this paper, we propose a new real-time optimistic protocol which employs the notion of dynamic adjustment of serialization order by backward-adjusting the non-serious conflicting transactions before the committing transactions so that many restarts can be eliminated. In the protocol, the overhead to support dynamic adjustment of serialization order is small. All checking is performed at the validation test of a transaction so as to provide more freedom for the adjustments. No conflict and serialization constraints have to be recorded during the read phase of a transaction. Performance studies of the proposed protocol by varying different parameters indicate that it outperforms other protocols.

Updating timestamp interval for dynamic adjustment of serialization order in Optimistic Concurrency Control-Time Interval (OCCTI) protocol

Updating timestamp interval for dynamic adjustment of serialization order in Optimistic Concurrency Control-Time Interval (OCCTI) protocol

Hybrid protocols using dynamic adjustment of serialization order for real-time concurrency control

Time-critical scheduling in real-time database systems has two components: real-time scheduling and concurrency control. While both concurrency control and real-time scheduling are well-developed and well-understood, there is only limited knowledge about the integration of concurrency control and real-time scheduling. Though recently the problem has been studied actively, the proposed solutions are still at an initial stage. A major source of problems in integrating the two is the lack of coordination in the development. They are developed on different objectives and incompatible assumptions (Buchmann 1989). Most of the proposed work for real-time concurrency control employ a simple method to utilize one concurrency control scheme such as 2PL, TO and OCC, and to consider the priority of operations inherited from the timing constraints of transactions in operation scheduling. This method has an inherent disadvantage of being limited by the concurrency control method used as the base. Since neither of pessimistic nor optimistic concurrency control is satisfactory by itself for real-time scheduling, this simple method using only one control can hardly satisfy the timing requirements of RTDBS. Problems such as excessive blocking, wasted restarts, and priority inversion are serious in RTDBS. In this paper, we proposed two real-time transaction scheduling protocols which employ a hybrid approach, i.e., a combination of both pessimistic and optimistic approaches. These protocols make use of a new conflict resolution scheme called dynamic adjustment of serialization order, which supports priority-driven scheduling, and avoids unnecessary aborts.