Global scheduling for flexible transactions in heterogeneousdistributed database systems

A heterogeneous distributed database environment integrates a set of autonomous database systems to provide global database functions. A flexible transaction approach has been proposed for the heterogeneous distributed database environments. In such an environment, flexible transactions can increase the failure resilience of global transactions by allowing alternate (but in some sense equivalent) executions to be attempted when a local database system fails or some subtransactions of the global transaction abort. We study the impact of compensation, retry, and switching to alternative executions on global concurrency control for the execution of flexible transactions. We propose a new concurrency control criterion for the execution of flexible and local transactions, termed F-serializability, in the error-prone heterogeneous distributed database environments. We then present a scheduling protocol that ensures F-serializability on global schedules. We also demonstrate that this scheduler avoids unnecessary aborts and compensation     

Efficient execution of read-only transactions in replicatedmultiversion databases

Multiple versions of data are used in database systems to increase concurrency. The higher concurrency results since read-only transactions can be executed without any concurrency control overhead and, therefore, read-only transactions do not interfere with the execution of update transactions. Availability of data in a distributed environment is improved by data replication. We propose a protocol for managing data in a replicated multiversion environment, where execution of read-only transactions or queries becomes completely independent of the underlying concurrency control and replica control mechanisms, and the data availability for read-only transactions increases significantly since they can be executed as long as any one copy of the object is available in the system. In order to validate the feasibility of our approach, we developed a simple prototype to measure the performance improvement in the response times of queries. The results clearly establish the viability of the approach as a useful paradigm for the design of efficient and fault-tolerant distributed database systems     

A non-two-phase locking protocol for global concurrency control indistributed heterogeneous database systems

A concurrency control method is proposed for global transactions in a distributed heterogeneous database system. This method is applicable when the database sites are interconnected in a rooted tree fashion. It guarantees deadlock freedom in addition to serializability. A general architecture of a heterogeneous system is given. The global transaction manager (GTM) decomposes the global transactions initiated at a site and the subtransactions received from other sites into smaller subtransactions, some of which are sent to the GTMs of the other sites, and those remaining, called g-local transactions, are to be executed by the local transaction manager (LTM) at that site. A concurrency control mechanism ensures serializability among: the local transactions (including the g-local transactions of the global ones) at each site, the global transactions and the global and local transactions together     

Real-time concurrency control in a multiprocessor environment

Although many high-performance computer systems are now multiprocessor-based, little work has been done in real-time concurrency control of transaction executions in a multiprocessor environment. Real-time concurrency control protocols designed for uniprocessor or distributed environments may not fit the needs of multiprocessor-based real-time database systems because of a lower concurrency degree of transaction executions and a larger number of priority inversions. This paper proposes the concept of a priority cap to bound the maximum number of priority inversions in multiprocessor-based real-time database systems to meet transaction deadlines. We also explore the concept of two-version data to increase the system concurrency level and to explore the abundant computing resources of multiprocessor computer systems. The capability of the proposed methodology is evaluated in a multiprocessor real-time database system under different workloads, database sizes and processor configurations. It is shown that the benefits of the priority cap in reducing the blocking time of urgent transactions are far greater than the losses involved in committing less urgent transactions. The idea of two-version data also greatly improves the system performance because of a much higher concurrency degree in the system     

移动分布式数据库中的实时事务处理

提出了移动事务实时提交协议(MTRTC)和多版本乐观并发控制协议(MVOCC)处理移动分布式实时事务。MVOCC有效性检查分为局部有效性检查和全局有效性检查,采用动态调整串行次序,避免了不必要的事务重启动,改善了只读事务的响应时间。MTRTC是一个实时提交协议,减少了通信信息。实验结果表明结合MVOCC和MTRTC事务处理协议优于其它事务处理协议。     

一种新的混合实时事务并发控制算法

单一的实时事务并发控制策略因为对事务性能以及事务对数据的访问方式有着特殊限制而无法满足不同类型事务同时并存的混合实时数据库的要求.针对不同类型实时事务特征,提出了一种新的混合实时事务并发控制算法,对不同类型实时事务采用不同并发控制策略,具有极强的针对性和自适应性,算法同时通过分析数据的相关语义,利用数据相似性定义,合理放宽可串行化的正确性标准,在优先考虑硬实时事务的前提下,尽可能增加软实时事务成功提交的比例以提高系统整体性能.仿真实验结果证明MRTT_CC算法性能良好.     

多库中并发控制的研究和实现

多库是一组分布在多个结点上自制的数据库系统的集合,每个局部数据库系统可能使用不同的并发控制协议。自制和异构为保证多库系统的全局可串性带来了困难。本文首先描述了多库中全局可串性问题,然后介绍了并发控制服务CCS系统的特点及系统的逻辑流程,最后讨论了系统中各个接口对象的设计及实现。     

Divergence control for distributed database systems

This paper presents distributed divergence control algorithms for epsilon serializability for both homogeneous and heterogeneous distributed databases. Epsilon serializability allows for more concurrency by permitting non-serializable interleavings of database operations among epsilon transactions. We first present a strict 2-phase locking divergence control algorithm and an optimistic divergence control algorithm for a homogeneous distributed database system, where the local orderings of all the sub-transactions of a distributed epsilon transaction are the same. In such an environment, the total inconsistency of a distributed epsilon transaction is simply the sum of those of all its sub-transactions. We then describe a divergence control algorithm for a heterogeneous distributed database system, where the local orderings of all the sub-transactions of a distributed epsilon transaction may not be the same and the total inconsistency of a distributed epsilon transaction may be greater than the sum of those of all its sub-transactions. As a result, in addition to executing a local divergence control algorithm in each site to maintain the local inconsistency, a global mechanism is needed to take into account the additional inconsistency Recommended by: Meichum Hsu     

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.     

P2P多数据库系统中的事务并发控制研究

P2P多数据库系统是一种分布式的数据库系统,它是P2P计算系统和传统数据库系统的结合体.P2P计算系统的动态性使得P2P多数据库系统的事务管理缺少全局控制机制.在事务传播过程不存在循环的前提下,文章提出了一种保证P2P多数据库系统中事务全局串行化的并发控制协议,它通过控制一系列全局事务传播过程中每个熟识关系的两级串行化来实现.     

一种新的实时数据库乐观并发控制方法

可串行化的并发控制对传统应用是合适的。而在实时数据库中，为了满足事务定时限制(典型地为截止期)，并且考虑到局部的数据库不一致能够随下一次数据采样恢复，人们提出了准一致可串行化标准。本文基于这一标准提出了一种新的乐观并发控制协议，它考虑了数据的相似性及事务特点，提高了事务执行的并发度，有利于实时事务定时限制的满足。     

Transaction Processing in a Mobile,Multi-Database Environment

Current distributed and multi-database systems are designed to allow timely and reliable access to large amounts of data distributed at different locations. Changes in current technology now allow users to access this data via a wide variety of devices through a diverse communication medium. A mobile data access system is an environment in which a wireless-mobile computing environment is superimposed upon a multi-database environment in order to realize anywhere, anytime access capability. As a potentially large number of users may siultaneously access the available data, there are several issues involved in the ability to concurrently manage transactions. Current multi-database concurrency control schemes do not efficiently manage these accesses because they do not address the limited bandwidth and frequent disconnections associated with wirelessnetworks.This paper first introduces the so-called mobile data access system (MDAS) and then proposes a new hierarchical concurrency control algorithm. The proposed concurrency control algorithm, v-lock, uses global locking tables created with semantic information contained within the hierarchy. The locking tables are subsequently used to serialize global transactions, and detect and remove global deadlocks. The performance of the new algorithm is simulated and the results are presented. In addition (through simulation) the performance of the proposed algorithm has been compared and contrasted against the site graph method, the potential conflict graph method, and the forced conflict method     

实时数据库系统中的并发控制协议及分析

实时数据库系统是事务和数据都具有显示的时间限制的数据库系统,与传统的数据库系统相比,其调度和并发控制策略、方法、机制有所不同。该文在分析实时数据库系统事务并发执行的新特点的基础上,介绍了几种具有代表意义的并发控制协议,总结了三种用于分析比较并发控制性能的方法,并分析了上述并发控制协议的性能。     

多数据库系统中的一致性维护

在多数据库环境中存在着全局事务和局部事务,它们必须相互协作,从而为应用程序提供一致性的数据。为了维护多数据库系统（MDBS）的一致性,提出一种事务管理机制。该机制基于扩展事务的原理,对全局子事务和局部事务进行扩展,并为各个局部数据库引进一些控制表。局部数据库管理系统（LDBMS）和局部数据库和控制表之上执行扩展务,这些扩展事务负一局可串行性及事务的恢复。由于它不依赖于LDBMS的各种特征,因此该方法易于实现,是实用的。     

Application Specific Transaction Management in Multidatabase Systems

We present a transaction model for multidatabase systems with autonomous component systems, coined heterogeneous 3-level transactions. It has become evident that in such a system the requirements of guaranteeing full ACID properties and full local autonomy can not be reconciled. In the heterogeneous 3-level transaction model, semantics of actions and specific properties of the architecture of multidatabase systems and of applications are utilized to define application specific compromises between these competing goals. We consider different definitions of conflicts between actions together with application specific sets of allowed (autonomous) local transactions and global subtransactions. A formal model for serializability is given. It is proven that heterogeneous 3-level transactions guarantee all consistency constraints that are ensured in serial executions. In contrast to most other models, we do not require data to be partitioned into global and local data. In particular, heterogeneous 3-level transactions even allow the same data to be updated by local and global transactions in parallel, if these updates are found to be semantically non-conflicting. Recovery is handled by inverse actions. We present a formal framework for this approach which is fully integrated into serializability theory by considering inverse actions as ordinary actions. This has an important impact on the relationship of concurrency control and recovery in our model: By changing the definition of conflicts between actions in order to tailor the model according to application needs, recovery is implicitly affected and no further explicit adaptation of recovery algorithms is necessary. The heterogeneous 3-level transaction model assumes component database systems to support ACID transaction properties. Further properties of local transactions or interfaces of the component database systems are not required. We describe a prototype implementation of heterogeneous 3-level transactions in the object-oriented database system VODAK.     

Transaction management issues in a failure-prone multidatabase system environment

This paper is concerned with the problem of integrating a number of existing off-the-shelf local database systems into a multidatabase system that maintains consistency in the face of concurrency and failures.The major difficulties in designing such systems stem from the requirements that local transactions be allowed to execute outside the multidatabase system control, and that the various local database systems cannot participate in the execution of a global commit protocol. A scheme based on the assumption that the component local database systems use the strict two-phase locking protocol is developed. Two major problems are addressed: How to ensure global transaction atomicity without the provision of a commit protocol, and how to ensure freedom from global deadlocks.     

Analysis of concurrency-coherency control protocols for distributedtransaction processing systems with regional locality

A system structure and protocols for improving the performance of a distributed transaction processing system when there is some regional locality of data reference are presented. A distributed computer system is maintained at each region, and a central computer system with a replication of all databases at the distributed sites is introduced. It provides the advantage of distributed systems principally for local transactions, and has the advantage of centralized systems for transactions accessing nonlocal data. Specialized protocols keep the copies at the distributed and centralized systems consistent without incurring the overhead and delay of generalized protocols for fully replicated databases. The advantages achievable through this system structure and the tradeoffs between protocols for concurrency and coherency control of the duplicate copies of the databases are studied. An approximate analytic model is used to estimate the system performance. It is found that the performance is sensitive to the protocol and that substantial performance improvement can be obtained as compared with distributed systems     

一种面向混合实时事务调度的并发控制协议

首先给出了一个两层结构的混合实时数据库系统模型,其中支持采用非定期任务调度算法来改进系统的性能．进一步,针对这种模型下混合事务的数据一致性问题,提出了一种新的并发控制协议——MCC-DATI．该协议采用动态优先级驱动的调度算法,通过限制非定期的软实时事务对硬实时事务的阻塞时间,保证硬实时事务的可调度性;同时,采用非定期任务调度算法以及基于时间戳间隔的动态串行化顺序调整机制来减少软实时事务的截止期错失率．仿真实验表明,相对于先前的混合事务的并发控制协议,该协议在不同的系统负载与截止期约束下都能够改进系统的性能。     

一种实时事务并发控制的新方法:SMVCC

提出了新颖的实时数据库事务并发控制方法：语义多版本并发控制协议（SMVCC）。利用实时数据库中存在多版本数据的特点,考虑实时数据和实时事务语义,结合实时数据库相似性概念,对传统多版本并发控制方法进行扩展,对不同事务采用不同并发控制策略,提高了事务并发度。     

分布式实时数据库并发控制

在集中式实时数据库中，事务并发控制一般采用基于优先级的事务重启或优先级继承．在分布式实时数据库中，由于一个主事务的各参与事务可能分布在多个节点上，事务重启一方面会造成系统资源的极大浪费，另一方面又会加大网络传输的负担．而在优先级继承策略中，被阻塞的高优先级事务的执行完全依赖于阻塞它的低优先级事务，在分布式环境中，事务的执行具有更多的不可确定的因素，这就很难保证被阻塞事务的截止期．本文提出了一种基于数据多映像的并发控制协议，事务因访问数据而形成不同的依赖关系，改变高优先级事务和低优先级事务的依赖关系从而更有利于高优先级事务而又不矢折或阻塞其他事务，从而大大提高事务执行的并发度，更好地满足实时事务的截止期．