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     

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     

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     

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。     

并行数据库实时多版本并发控制协议性能研究

随着对实时数据库事务处理性能要求的不断提高 ,将并行数据库和实时数据库相结合的并行实时数据库将成为新的发展方向 .然而通常的实时多版本并发控制协议不适合并行数据库的无共享结构 .比较了几种并发控制协议在无共享结构下的性能 ,该文提出了一种并行实时多版本并发控制协议 .它能有效地减少事务的重启 ,降低事务的错过率 .在长短事务混合的情况下 ,它的性能比其它的并发控制协议更好 .同时 ,它具有更好的可扩展性 .作者利用仿真模型对该协议进行了性能测试 ,与其它协议进行了比较 ,并分析了实验结果 ,仿真实验结果表明该协议性能良好     

移动实时事务的相对时间正确性及其实现

随着移动通信技术的迅速发展,人们提出了新的应用要求：在移动环境下处理实时事务．而移动通信的带宽有限性引起较大的数据访问延迟,有时甚至由于网络传输的断接而使得事务得不到所需访问的数据,这些严重影响了移动环境下实时事务截止期的满足．实时事务的时间要求(典型地为截止性)的满足通常比事务执行的逻辑结果的完全正确更为重要．该文提出了移动环境下实时事务的“相对”时间正确性标准,只要事务访问的数据的陈旧度(staleness)在控制的范围内,就不一定要求实时事务访问的数据在事务提交时是有效的．文中提出了移动实时事务的相对时间正确性标准、保证事务相对时间正确性标准的算法及移动实时事务执行模型．所有这些策略一方面降低了移动实时事务的并发控制代价,另一方面以牺牲事务的“绝对”时间正确换取事务截止期的满足,这与实时数据库“宁愿要部分正确但及时的结果而不愿要绝对正确但过时的结果”的系统目标是相一致的．实验表明：相对时间正确性标准的提出及相应的事务执行模型能提高系统中满足截止期事务的比率,从而提高了系统性能．     

新的实时乐观并发控制协议的研究

提出了一种新的基于事务重要性的实时乐观并发控制协议。其基本思想是当系统中事务属不同类时,有一些事务的重要性要大于其它事务,在冲突消解中利用关于事务重要性的信息来减少重要性大的事务被延误的数量,提高并发控制机制性能。通过模拟试验分析该协议,表明其能有效地减少重要性大事务的失效率,但对总体事务的失效率影响不大。     

并行实时数据库系统中的一种自适应并发控制模式

根据事务的实时性及关键性分类,事务分为硬实时、固实时、软实时和非实时事务。传统的并发控制仅考虑其中一到两种事务,且将固／软实时事务统一为软实时事务考虑,带有很大的不精确性,这将降低系统吞吐率。为此,本文提出了一种自适应并发控制模式。该模式采用两级并发控制体系,一级是主并发控制器,探测不同类事务间可能存在
在的类间数据冲突;二级是从并发控制器,探测同类事务间可能存在的类内数据冲突,由四类从并发控制器实现。根据各类事务的不同特征,各控制器采用的策略不同,因而具有极大的自适应性并能最终提高系统的吞吐率,但需要增加额外的硬件开销。     

Real-time transaction scheduling in database systems

A database system supporting a real-time application, which can be called “a real-time database system (RTDBS)”, has to provide real-time information to the executing transactions. Each RTDB transaction is associated with a timing constraint, usually in the form of a deadline. Efficient resource scheduling algorithms and concurrency control protocols are required to schedule the transactions so as to satisfy both timing constraints and data consistency requirements. In this paper,^† we concentrate on the concurrency control problem in RTDBSs. Our work has two basic goals: real-time performance evaluation of existing concurrency control approaches in RTDBSs, and proposing new concurrency control protocols with improved performance. One of the new protocols is locking-based, and it prevents the priority inversion problem^{, †} by scheduling the data lock requests based on prioritizing data items. The second new protocol extends the basic timestamp-ordering method by involving real-time priorities of transactions in the timestamp assignment procedure. Performance of the protocols is evaluated through simulations by using a detailed model of a single-site RTDBS. The relative performance of the protocols is examined as a function of transaction load, data contention (which is determined by a number of system parameters) and resource contention. The protocols are also tested under various real-time transaction processing environments. The performance of the proposed protocols appears to be good, especially under conditions of high transaction load and high data contention.     

Transaction decomposition using transaction semantics

Serializability has been widely accepted as the correctness criterion for databases subject to concurrent access. However, in a number of the newer and most challenging application areas, serializable execution may not be feasible.

? Serializable execution is generally implemented using a two-phase locking algorithm that locks items in the database to delay transactions that are in danger of performing in a non-serializable fashion. Such delays are unacceptable in high performance database systems that must process hundreds, and perhaps thousands, of transactions per second and in systems supporting long-running transactions.

? In systems in which data is distributed across a federated database, a global transaction is decomposed into a set of local subtransactions executed at a subset of the sites. Serializable execution in such systems not only incurs a performance penalty, but also requires the component systems to cooperate (for example in a two phase commit protocol), with a resulting loss of site autonomy. In many applications, the component systems either can not or will not agree to the required cooperation.

A number of models have recently been proposed in which transactions are decomposed into smaller, atomic, interleavable steps. These models have the potential for improving performance since locks are released at the end of each step. Models of distributed transactions have also been proposed with the similarity that subtransactions correspond to steps. In most of this work serializability is no longer guaranteed. In this paper we propose a new, application-oriented, correctness criterion that utilizes transaction semantics. We treat transactions as programs whose semantics can be analyzed at design time. The effect of each transaction is specified using pre- and postconditions, and any schedule that preserves these conditions is permissible. Such schedules can produce database states that could not be reached by any serial execution. In addressing the issue of performance, we use transaction semantics to decompose transactions into steps and describe a concurrency control that controls step interleaving in such a way that assertions are preserved. The same model can be used to study the interleaving of subtransactions of concurrent distributed transactions.     

基于级联深度的主动实时乐观并发控制协议

主动实时数据库因结合了时间限制与主动机制而使系统事务的并发控制变得更为复杂。主动规则的引入使事务触发新的事务且在执行上具有多种耦合方式,传统的实时并发控制策略无法对具有复杂执行模式的事务进行有效调度,而基于主动数据库的并发控制机制也没有考虑事务的实时性问题。通过对事务不同耦合方式的实时要求及事务间冲突关系进行分析,提出了新的主动实时数据库乐观并发控制方法,对不同事务级联深度进行评估,结合事务执行的时间信息对冲突事务进行动态调整串行化顺序。理论分析与实验证明,能在保证事务可串行性的同时降低了不必要事务重启个数,更好地满足系统的实时性。     

Performance analysis of dynamic locking with the no-waiting policy

A transaction processing system with two-phase dynamic locking with the no waiting policy (DLNW) for concurrency control is considered. In this method, transactions making conflicting lock requests are aborted and restarted rather than blocked, thereby eliminating blocking delays (and deadlocks), but making it susceptible to cyclic restarts. Cyclic restarts are dealt with by delaying the restart of a transaction encountering a lock conflict or replacing it with a new transaction. Analytic solution methods for evaluating the performance of the variants of the DLNW method are described. The analytic methods, validated against simulation and shown to be acceptably accurate, are used to study the effect of the following parameters on system performance: transaction size and its distribution, degree of concurrency, the throughput characteristic of the computer system, and the mixture of read-only query and update transactions. A comparison of the DLNW and dynamic locking with waiting (DLW) methods shows that DLW provides higher throughput than DLNW, except when there is no hardware resource contention and conflicted transactions can be replaced by new transactions. The DLNW method outperforms the time-stamp ordering method, as observed from simulation results as well as case by case analyses of possible scenarios     

Effects of shaping characteristics on the performance of nested transactions

The nested transaction model was introduced to satisfy the requirements of advanced database applications. Moreover, it is currently the basic transaction model for new database applications like workflow systems and new database systems like mobile databases and object-relational databases. Though there are several performance evaluation studies of different concurrency control mechanisms in nested transactions, the effects of transaction parameters on the overall system performance have not received any attention. In this paper, we study the effects of transactions characteristics on system performance. We developed a detailed simulation model and conducted several experiments to measure the impact of transactions characteristics on the performance. First, the effect of the number of leaves on the performance of nested transactions is investigated under different shaping parameters. Also, effects of the depth of the transaction tree on the system performance are investigated.     

Read-down conflict-preserving serializability as a correctness criterion for multilevel-secure optimistic concurrency control: CSR/RD

It is important to note that conflict-preserving serializability and related theory are artifacts of an era of database development where correctness alone was the overriding concern. With the advent of multilevel-secure databases, there is clearly a need to reexamine such theories. Any correctness criterion to govern transaction processing in the multilevel security context has to incorporate both secureness and correctness in a unified manner. This paper makes original contributions in two different but closely related areas to the optimistic concurrency control in multilevel-secure, single-version databases. First, read-down conflict-preserving serializability (CSR/RD) captures multilevel-secure database consistency requirements and secure transaction correctness properties via a single notion. Second, it presents a multilevel-secure optimistic concurrency control (MLS/OCC) scheme that has several desirable properties: If lower-level transactions were somehow allowed to continue with their executions in spite of the conflict with high-level transactions, covert timing-channel freeness would be satisfied. This sort of optimistic approach for conflict insensitiveness and the properties of non-blocking and deadlock freedom make the optimistic concurrency control scheme especially attractive to multilevel-secure transaction processing.     

基于验证因子的实时多版本并发控制

为了解决乐观并发控制方法的不必要重启与即将超过截止期事务的重启引发的负载问题,提出了验证因子的概念及基于验证因子的实时多版本并发控制方法（MVOCC-VF）．通过对验证因子的检测,优先调度完成度较高的事务;同时结合多版本技术增强事务调度的并发度,避免了更多的不必要重启问题,尤其能有效保证即将超截止期事务的完成,并证明了协议的正确性．实验结果表明：与已有的实时并发控制方法相比,MVOCC—VF降低了事务重启个数及超截止期百分率,提高了系统的总体性能．     

User‐level operating system transactions

User‐level operating system transactions allow system administrators and ordinary users to perform a sequence of file operations and then commit them as a group, or abort them without leaving any trace behind. Such a facility can aid many system administration and software development tasks. The snapshot isolation concurrency control mechanism allows transactions to be implemented without locking individual system calls; conflicts are detected when the transaction is ready to commit. Along these lines we have implemented a user‐space transaction monitor that is based on ZFS snapshots and a file system event monitor. Transactions are committed through a robust and efficient algorithm that merges the operations performed on a file system's clone back to its parent. Both the performance impact and the implementation cost of the transaction monitor we describe are fairly small. Copyright © 2009 John Wiley & Sons, Ltd.     

基于耦合模式的主动实时并发控制算法

主动实时数据库中触发事务与被触发事务在执行上具有多种耦合模式,传统的并发控制无法对具有复杂耦合模式的事务进行有效调度。通过对不同耦合模式实时要求及事务间冲突关系的分析,提出了新的主动实时并发控制算法(ARTCC-CM),采用时戳区间策略,在验证阶段检测冲突事务触发度及执行时间,动态调整串行化顺序。理论分析与实验证明,在保证可串行性同时减少了不必要的事务重启,提高了系统性能。     

Formal aspects of optimistic concurrency control in a multiple version database system

Formal aspects of optimistic concurrency control in a multiple version database system are discussed. It is distinguished between element-optimistic and set-optimistic methods. In an element-optimistic method the unit of scheduling for the concurrency control is one action/transaction, while in a set-optimistic method a set of transactions is scheduled. The underlying decision problems of serializability are defined and shown to be NP-complete in a model, which is typical for most modern transaction oriented database management systems. Therefore it is most probable, that both optimistic concurrency control types cannot be implemented efficiently in the general case.     

基于准一致性可串行化的并发控制协议

人们基于准一致性可串行化的正确性标准，提出了一种乐观并发控制协议，提高了事务执行的并发度，更有利于实时事务定时限制的满足。