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     

事务处理原子性保证策略

本文主要讨论了多数据库系统中在全局事务取消或失败的情况下,如何保证事务原子性的问题,在分析重做,重试,补偿方法和两阶段提交协议这几种方法的利,弊的基础上,充分利用它们的优点,在一个系统中实现了四种方法的有机结合。     

分布式实时事务提交处理

由于提交处理的复杂性,分布式实时事务很难满足其截止期.提出了一种新的提交协议A2SC(主动的双空间提交),它适合于分布式实时事务提交处理的需要.分析了由于数据冲突访问而形成的各种依赖关系.当处于准备状态的事务和处于提交状态的事务发生数据冲突访问时,A2SC允许处于执行状态的事务在一种控制的方式下乐观地访问锁住的数据.当处于准备状态的事务夭折时,仅仅只有其夭折依赖集中的事务夭折.进一步提出了"没有结果的运行"的观念.当一个事务发现它是没有结果的允许时,它将主动夭折.进行了广泛的模拟实验比较A2SC和其它协议比如基准协议、PROMPT和DDCR的性能.模拟结果表明A2SC在最小化错过截止期的事务数方面较成功,因此A2SC适合于高性能分布式实时事务.     

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.     

Transaction processing in a peer to peer database network

This paper investigates a transaction processing mechanism in a peer to peer database network. A peer to peer database network is a collection of autonomous data sources, called peers, where each peer augments a conventional database management system with an inter-operability layer (i.e. mappings) for sharing data. In this network, each peer independently manages its database and executes queries as well as updates over the related data in other peers. In this paper, we consider a peer to peer database network where mappings between peers are established through data-level mappings for sharing data and resolving data heterogeneity. With regards to transaction processing in a peer to peer database network, we mainly focus on how to maintain a consistent execution view of concurrent transactions in peers without a global transaction coordinator. Since there is no global transaction coordinator and each peer executes concurrent transactions independently, different peers may produce different execution views for the same set of transactions. For this purpose, we investigate potential problems that arise when maintaining a consistent execution of concurrent transactions. In order to guarantee consistent execution, we introduce a correctness criteria and propose two approaches, namely Merged Transactions and OTM based propagation. We assume that one single peer initiates the concurrent transactions. We also present a solution for ensuring the consistent execution view of concurrent transactions considering the failures of 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。     

移动分布式实时嵌套事务提交

在移动分布式计算环境中,事务移动性和无线网络固有的缺陷使得传统的分布式实时事务管理机制不足以支持移动分布式实时事务的执行,故有必要为移动实时事务研究新的事务处理机制,以提高其成功率.着重研究移动实时事务的提交机制.首先,通过分析移动分布环境中实时事务的特点给出了一个基于功能替代的移动实时嵌套事务模型.然后,提出了一个基于此模型的三层提交结构以及能够保证移动实时事务原子性和结构正确性的三阶段实时提交协议3PRTC(three-phase real-time commit).性能测试表明,所提出的事务模型及其提交机制能够提高实时事务的成功率.     

Revisiting Transaction Management in Multidatabase Systems

A lot of research efforts have focused on global serializability, global atomicity, and global deadlocks in multidatabase systems. Surprisingly, however, very few transaction processing model exists that ensures global serializability, global atomicity, and freedom from global deadlocks in a uniform manner. In this paper, we examine previous transaction processing models and propose a new transaction processing model that generates globally serializable and deadlock-free schedules in failure-prone multidatabase systems. A new transaction processing model adopts rigid conflict serializability as a correctness criterion on global serializability, and follows an emulated 2PC, criteria for global commitment, and an abort-based multidatabase recovery scheme for global serializability in failure-prone multidatabase systems. In addition, a deadlock-free policy is suggested where rigid conflict serializability is enforced when each subtransaction, including redo transactions, begins its execution. To practically support a new transaction processing model, Rigid Ticket Ordering (RTO) methods are designed. The proposed transaction processing model has the following improvements: (a) it resolves abnormal direct conflicts identified in this paper, (b) it imposes no restrictions on the execution of local transactions, and (c) it relaxes the restrictions on the execution of global transactions.     

Recovery of PTUIE handling from source codes through recognizing its probable properties

Automated recovery of system features and their designs from program source codes is important in reverse engineering and system comprehension. It also helps in the testing of software. An error that is made by users in an input to an execution of a transaction and discovered only after the completion of the execution is called a posttransaction user-input error (PTUIE) of the transaction. For a transaction in any database application, usually, it is essential to-provide transactions for correcting the effect that could result from any PTUIE of the transaction. We discover some probable properties that exist between the control flow graph of a transaction and the control flow graphs of transactions for correcting PTUIE of the former transaction. Through recognizing these properties, we present a novel approach for the automated approximate recovery of provisions and designs for transactions to correct PTUIE of transactions in a database application. The approach recognizes these properties through analyzing the source codes of transactions in the database application statically.     

On Concurrency Control in Multidatabase Systems with an Extended Transaction Model

A major concurrency control problem that we have to cope in multidatabase systems is the global deadlock detection and resolution problem. This detection must take into account the autonomy of local systems, which make impossible the visibility of the state of local transactions. A well-known approach to detect such deadlocks, called potential global deadlocks, is one based on the potential conflict graph (PCG) appropriate for the multidatabase transaction model with a global commit protocol. This classical transaction model is very constraining for applications manipulating great volumes of information, and where subtransaction terminations (commit or abort) of global transactions are not totally dependant. In this paper we present an effective potential global deadlock characterization, and an efficient potential global deadlock detection algorithm, in multidatabase systems with an extended transaction model more suited for such applications.     

移动数据库系统乐观事务处理策略

在移动数据库系统中，计算平台的移动性，频繁的断接性以及长事务等特性使得传统的事务处理模式不再适用，为了解决移动数据中的事处处理问题，提出了一种新的移动事务处理模型－乐观两阶段提交移动事务模型（O2PC－MT），该模型采用乐观并发控制与两阶段提交协议相结合的方法，对移动事务的长事务特性提供了灵活与有效的支持，此外，该模型允许移动计算机分多发送中务操作，且在事务执行过程中可以任意移动，从而提供了对交互式事务及随意移动性的支持，实验结果表明，与基于两段锁协议及其变形的其它移动事务处理模型相比，O2PC－MT提高了系统的事务吞吐率并改善了系统的总体性能。     

Transaction recovery in federated autonomous databases

Transaction management for federated database systems (FDBSs), where each participating DBMS is autonomous, supports global transactions that can access more than one database. A number of concurrency control algorithms exist for the failure-free environment. Handling transaction failure, due to concurrency control or site related reasons, becomes especially difficult in the presence of autonomy. Due to local autonomy, carrying out 2-phase commit may not be possible. This can be simulated by providing a server on top of the pre-existing DBMS at each site, which is responsible for submitting the local operations to the associated DBMS and communicating with the transaction's originating site. In this paper we formalize the problem of ensuring transaction consistency in an FDBS environment in the presence of failure. The key problem is that due to autonomy, the local DBMS and FDBS may have different views of an execution sequence generated at a site.Local recoverability is identified as the property of local execution sequences necessary for correctness. The other main problem is of guaranteeing that the various locally recoverable histories are consistent with each other. These identified properties are necessary and sufficient conditions for the correctness of FDBS recovery algorithms, and can be used to evaluate the correctness of the proposed algorithms. This paper also presents an FDBS recovery algorithm that has been proved to be correct. Formal proofs of all properties and a comparison of different algorithms are provided. Recommended by: Y. BreitbartThis work was supported in part by sub-contract #B09530013 from Honeywell SSDC, under contract #F30602-91-C-0128 from Rome Laboratory of the US Air Force.On leave from Computer Science Department, Heilongjiang University, People's Republic of China.     

Transaction Fusion: A Model for Data Recovery from Information Attacks

The escalation of electronic attacks on databases in recent times demands fast and efficient recovery methods. The existing recovery techniques are too time-consuming as they first undo all malicious and affected transactions individually, and then redo all affected transactions, again, individually. In this paper, we propose a method that accelerates the undo and redo phases of the recovery. The method developed involves combining or fusing malicious or affected transactions occurring in groups. These fused transactions are executed during undo and redo phases instead of execution of individual transactions. By fusing relevant transactions into a single transaction, the number of operations such as start, commit, read, and write are minimized. Thus, data items which were required to be accessed multiple times in case of individual transactions are accessed only once in a fused transaction. The amount of log I/O's is reduced. This expedites the recovery procedure in the event of information attacks. A simulation analysis of the proposed model confirmed our claim.     

Applying semantic knowledge to real-time update of access control policies

Real-time update of access control policies, that is, updating policies while they are in effect and enforcing the changes immediately, is necessary for many security-critical applications. In this paper, we consider real-time update of access control policies in a database system. Updating policies while they are in effect can lead to potential security problems, such as, access to database objects by unauthorized users. In this paper, we propose several algorithms that not only prevent such security breaches but also ensure the correctness of execution. The algorithms differ from each other in the degree of concurrency provided and the semantic knowledge used. Of the algorithms presented, the most concurrency is achieved when transactions are decomposed into atomic steps. Once transactions are decomposed, the atomicity, consistency, and isolation properties no longer hold. Since the traditional transaction processing model can no longer be used to ensure the correctness of the execution, we use an alternate semantic-based transaction processing model. To ensure correct behavior, our model requires an application to satisfy a set of necessary properties, namely, semantic atomicity, consistent execution, sensitive transaction isolation, and policy-compliant. We show how one can verify an application statically to check for the existence of these properties.     

实时Client/Server数据库并发控制和恢复机制研究

该文提出了实时Client／Server数据库系统多版本两阶段封锁并发控制协议和有效的恢复机制。协议区分只读事务和更新事务。只读事务在执行读操作时遵从多版本时间排序协议,更新事务执行强两阶段封锁协议,即持有全部锁直到事务结束。只读事务读请求从不失败,不必等待等特性。在典型数据库系统中,读操作比写操作频繁。这个特性对于实践来说至关重要。为了提高只读事务的响应时间,协议让每个客户端与一个一致数据库影子相联,只读事务在客户端处理。更新事务提交到服务端运行。服务端每个事务Ti在提交时系统必须向所有客户端广播信息。客户端根据得到的广播信息自动构造一致数据库影子。一致数据库影子还将用于系统恢复。通过仿真模拟。与2V2PL和OCC-TI-WAIT-50协议进行比较,结果表明：该并发控制协议不仅能有效降低事务延误截止时间率和重起动率,而且能改善只读事务的响应时间,减少优先级高事务的锁等待时间。协议性能优于2V2PL协议和OCC-TI-WAIT-50协议。     

A Commit Strategy for Distributed Real-Time Transaction

Ramamritham gives three common types of constraints for the execution his-tory 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, be-cause of data access conflicts, axe analyzed. Based on the analysis, an optimistic commit protocol 2LC (two-Level Commit) is proposed, which is specially designed for the distributed real-time do-main. 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 ex-periments 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 num-ber of missed transaction deadlines. Furthermore, it is easy to be incorporated with the existing concurrency control protocols.     

On rigorous transaction scheduling

The class of transaction scheduling mechanisms in which the transaction serialization order can be determined by controlling their commitment order, is defined. This class of transaction management mechanisms is important, because it simplifies transaction management in a multidatabase system environment. The notion of analogous execution and serialization orders of transactions is defined and the concept of strongly recoverable and rigorous execution schedules is introduced. It is then proven that rigorous schedulers always produce analogous execution and serialization orders. It is shown that the systems using the rigorous scheduling can be naturally incorporated in hierarchical transaction management mechanisms. It is proven that several previously proposed multidatabase transaction management mechanisms guarantee global serializability only if all participating databases systems produce rigorous schedules     

Speculative locking protocols to improve performance for distributed database systems

We have proposed speculative locking (SL) protocols to improve the performance of distributed database systems (DDBSs) by trading extra processing resources. In SL, a transaction releases the lock on the data object whenever it produces corresponding after-image during its execution. By accessing both before and after-images, the waiting transaction carries out speculative executions and retains one execution based on the termination (commit or abort) mode of the preceding transactions. By carrying out multiple executions for a transaction, SL increases parallelism without violating serializability criteria. Under the naive version of SL, the number of speculative executions of the transaction explodes with data contention. By exploiting the fact that a submitted transaction is more likely to commit than abort, we propose the SL variants that process transactions efficiently by significantly reducing the number of speculative executions. The simulation results indicate that even with manageable extra resources, these variants significantly improve the performance over two-phase locking in the DDBS environments where transactions spend longer time for processing and transaction-aborts occur frequently.     

分布式实时事务提交协议

在分布式实时数据库系统中，保证事务原子性的唯一途径是研究和开发出一个实时的原子提交协议．首先详细分析了事务因数据访问冲突而形成的各种依赖关系，在此基础上提出了实时的原子乐观提交协议——2SC协议，该协议减少了事务的等待时间，提高了事务的并发度，且能无缝地和现有的并发控制协议集成在一起，保证事务的可串行化和原子性．通过模拟实验研究表明，采用该协议能够减少超过截止期的事务数目。     

SWIFT—A new real time commit protocol

Although there are several factors contributing to the difficulty in meeting distributed real time transaction deadlines, data conflicts among transactions, especially in commitment phase, are the prime factor resulting in system performance degradation. Therefore, design of an efficient commit protocol is of great significance for distributed real time database systems (DRTDBS). Most of the existing commit protocols try to improve system performance by allowing a committing cohort to lend its data to an executing cohort, thus reducing data inaccessibility. These protocols block the borrower when it tries to send WORKDONE/PREPARED message [1, 6, 8, 9], thus increasing the transactions commit time. This paper first analyzes all kind of dependencies that may arise due to data access conflicts among executing-committing transactions when a committing cohort is allowed to lend its data to an executing cohort. It then proposes a static two-phase locking and high priority based, write-update type, ideal for fast and timeliness commit protocol i.e. SWIFT. In SWIFT, the execution phase of a cohort is divided into two parts, locking phase and processing phase and then, in place of WORKDONE message, WORKSTARTED message is sent just before the start of processing phase of the cohort. Further, the borrower is allowed to send WORKSTARTED message, if it is only commit dependent on other cohorts instead of being blocked as opposed to [1, 6, 8, 9]. This reduces the time needed for commit processing and is free from cascaded aborts. To ensure non-violation of ACID properties, checking of completion of processing and the removal of dependency of cohort are required before sending the YES-VOTE message. Simulation results show that SWIFT improves the system performance in comparison to earlier protocol. The performance of SWIFT is also analyzed for partial read-only optimization, which minimizes intersite message traffic, execute-commit conflicts and log writes consequently resulting in a better response time. The impact of permitting the cohorts of the same transaction to communicate with each other [5] on SWIFT has also been analyzed. Recommended by: Ahmed Elmagarmid