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.     

An efficient deadlock prevention approach for service oriented transaction processing

Transaction processing can guarantee the reliability of business applications. Locking resources is widely used in distributed transaction management (e.g., two phase commit, 2PC) to keep the system consistent. The locking mechanism, however, potentially results in various deadlocks. In service oriented architecture (SOA), the deadlock problem becomes even worse because multiple (sub)transactions try to lock shared resources in the unexpectable way due to the more randomicity of transaction requests, which has not been solved by existing research results. In this paper, we investigate how to prevent local deadlocks, caused by the resource competition among multiple sub-transactions of a global transaction, and global deadlocks from the competition among different global transactions. We propose a replication based approach to avoid the local deadlocks, and a timestamp based approach to significantly mitigate the global deadlocks. A general algorithm is designed for both local and global deadlock prevention. The experimental results demonstrate the effectiveness and efficiency of our deadlock prevention approach. Further, it is also proved that our approach provides higher system performance than traditional resource allocation schemes.     

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.     

Global committability in multidatabase systems

Develops a formal basis for research into the reliability aspects of transaction processing in multidatabase systems (MDBSs). We define a new correctness notion called `global committability' for the correct unilateral commit and the retry recovery of global transactions in an autonomous MDBS environment. This notion makes it easier to ensure the isolation property of global transactions when the retry approach is applied. The formalization work illustrates that the conventional serializability and recoverability notions are not sufficient to specify the correct execution (i.e. isolated execution and recovery) of global transactions when the unilateral commit and the retry recovery are used to ensure the atomicity of global transactions. This work is significant because the unilateral commit and the retry recovery are an attractive complementary means to the undo recovery (whose correct schedule is specified by the conventional recoverability notion) for advanced transaction applications with the characteristics of site autonomy and long-lived execution     

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.     

Using tickets to enforce the serializability of multidatabasetransactions

To enforce global serializability in a multidatabase environment the multidatabase transaction manager must take into account the indirect (transitive) conflicts between multidatabase transactions caused by local transactions. Such conflicts are difficult to resolve because the behavior or even the existence of local transactions is not known to the multidatabase system. To overcome these difficulties, we propose to incorporate additional data manipulation operations in the subtransactions of each multidatabase transaction. We show that if these operations create direct conflicts between subtransactions at each participating local database system, indirect conflicts can be resolved even if the multidatabase system is not aware of their existence. Based on this approach, we introduce optimistic and conservative multidatabase transaction management methods that require the local database systems to ensure only local serializability. The proposed methods do not violate the autonomy of the local database systems and guarantee global serializability by preventing multidatabase transactions from being serialized in different ways at the participating database systems. Refinements of these methods are also proposed for multidatabase environments where the participating database systems allow schedules that are cascadeless or transactions have analogous execution and serialization orders. In particular, we show that forced local conflicts can be eliminated in rigorous local systems, local cascadelessness simplifies the design of a global scheduler, and that local strictness offers no significant advantages over cascadelessness     

多数据库事务处理模型中局部代理的设计与实现

在多数据库事务处理模型中，局部代理提供了全局事务管理层与局部数据库系统的接口，它使多数据库系统能够对分布在不同站点的局部数据库进行透明访问。文中提出了基于两阶段代理（２ＰＣＡ）的局部代理模型，它集成了支持两阶段提交或单阶段提并的数据库系统、以及并不支持事务提交协议的文件系统。文中还介绍了采用专用接口和ＯＤＢＣ两种实现局部代理的方法。文中着重介绍了使用ＯＤＢＣ方法的具体实现，并对两种方法实现的局部代     

Overview of multidatabase transaction management

A multidatabase system (MDBS) is a facility that allows users access to data located in multiple autonomous database management systems (DBMSs). In such a system,global transactions are executed under the control of the MDBS. Independently,local transactions are executed under the control of the local DBMSs. Each local DBMS integrated by the MDBS may employ a different transaction management scheme. In addition, each local DBMS has complete control over all transactions (global and local) executing at its site, including the ability to abort at any point any of the transactions executing at its site. Typically, no design or internal DBMS structure changes are allowed in order to accommodate the MDBS. Furthermore, the local DBMSs may not be aware of each other and, as a consequence, cannot coordinate their actions. Thus, traditional techniques for ensuring transaction atomicity and consistency in homogeneous distributed database systems may not be appropriate for an MDBS environment. The objective of this article is to provide a brief review of the most current work in the area of multidatabase transaction management. We first define the problem and argue that the multidatabase research will become increasingly important in the coming years. We then outline basic research issues in multidatabase transaction management and review recent results in the area. We conclude with a discussion of open problems and practical implications of this research.     

PCI总线死锁机制的研究

滞后事务与延迟事务是ＰＣＩ系统用来改善性能的两种特殊处理事务的方式,但须保证事务的顺序性、数据的一致性,防止死锁。本文研究了ＰＣＩ总线死锁的产生机制,提出了防止措施。     

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。     

On Characterization and Correctness of Distributed Deadlock Detection

Distributed deadlock detection requires identifying the presence of certain properties in the global state of distributed systems. Distributed deadlock detection is complicated due to the lack of both global memory and a common physical clock, and due to unpredictable message delays. We characterize the formation and detection of distributed deadlocks in terms of the contents of local memory of distributed nodes/sites. We describe how the interaction between deadlock detection and deadlock resolution can lead to the detection of false deadlocks that are impossible to avoid due to inherent system limitations. We define shadow, phantom, and pseudo deadlocks in the proposed framework. We give examples of existing incorrect deadlock detection algorithms to illustrate how they violate the developed requirements for distributed deadlock detection. The characterization provides an insight into the properties of distributed deadlocks, expresses inherent limitations of distributed deadlock detection, and yields new correctness criteria for distributed deadlock detection algorithms.     

Deadlock Models and a General Algorithm for Distributed Deadlock Detection

This paper deals with the problem of deadlock detection in asynchronous message passing systems in a system model that covers unspecified receptions and non-FIFO channels. It presents a hierarchy of deadlock models and deadlock detection problems. It abstracts deadlocks by a general deadlock model that has the same modeling power as the OR-AND model; however, it has much concise expressive power. An abstract general definition of deadlocks in distributed systems is presented that defines deadlocks independently of the underlying deadlock model. This formulation can be used to design a single distributed deadlock detection algorithm which uniformly addresses all deadlocks in the context of various request models such as AND, OR, AND-OR, and k-out-of-n requests. A simple generalized deadlock detection algorithm that uses a circulating token is presented to illustrate the concept. The algorithm is formally described and proven correct. Moreover, possible refinements of the basic solution concerning improvements of token routing and parallel implementation are outlined and evaluated. Extensions to individual and global termination issues are also addressed. Since the proposed deadlock detection algorithm is designed around the abstract definition of deadlocks, it has some very favorable features.     

Cooperative transaction hierarchies: Transaction support for design applications

Traditional atomic and nested transactions are not always well-suited to cooperative applications, such as design applications. Cooperative applications place requirements on the database that may conflict with the serializability requirement. They require transactions to be long, possibly nested, and able to interact with each other in a structured way. We define a transaction framework, called acooperative transaction hierarchy, that allows us to relax the requirement for atomic, serializable transactions to better support cooperative applications. In cooperative transaction hierarchies, we allow the correctness specification for groups of designers to be tailored to the needs of the application. We usepatterns andconflicts to specify the constraints imposed on a group's history for it to be correct. We also provide some primitives to smooth the operation of the members. We characterize deadlocks in a cooperative transaction hierarchy, and provide mechanisms for deadlock detection and resolution. We examine issues associated with failure and recovery.     

A Priority Based Distributed Deadlock Detection Algorithm

Deadlock handling is an important component of transaction management in a database system. In this paper, we contribute to the development of techniques for transaction management by presenting an algorithm for detecting deadlocks in a distributed database system. The algorithm uses priorities for transactions to minimize the number of messages initiated for detecting deadlocks. It does not construct any wait-for graph but detects cycles by an edge-chasing method. It does not detect any phantom deadlock (in the absence of failures), and for the resolution of deadlocks it does not need any extra computation. The algorithm also incorporates a post-resolution computation that leaves information characterizing dependence relations of remaining transactions of the deadlock cycle in the system, and this will help in detecting and resolving deadlocks which may arise in the future. An interesting aspect of this algorithm is that it is possible to compute the exact number of messages generated for a given deadlock configuration. The complexity is comparable to the best algorithm reported. We first present a basic algorithm and then extend it to take into account shared and exclusive lock modes, simultaneous acquisition of multiple locks, and nested transactions.     

An extended transaction to maintain consistency and recovery in multidatabase systems

A complete practical solution to transaction management preserving multidatabase consistency in the presence of multidatabase updates and failures is presented. The approach developed does not require support for the two-phase commit (2PC) protocol in the participating local database management systems (LDBMSs). Furthermore, it does not violate local autonomy; the source code of the LDBMSs is not modified in any way and the multidatabase system (MDBS) does not access or modify any control information of the LDBMS. The principles of the 2PC protocol in the process of global transaction commitment are adopted. The presented method does not rely on any specific concurrency control mechanism for LDBMSs. Consideration is given to global transaction failures due to subtransaction aborts by the LDBMSs and local site crashes. The recovery process is based on undo operations. While a global transaction is in progress, the tables accessed by subtransactions of this transaction at each local site are locked using specially initiated table locks. These locks are stored and maintained in the local database itself as control tables. The approach taken is easy to implement, and its limitations are discussed.     

基于事务语义的多数据库系统并发存取控制方法

本文提出了一种多数据库系统（MDBS）的基于事务语义的并发存取控制方法。文中首先按对数据库一致性的影响将事务分类;接着提出了新的MDBS的并发调度的非可串行正确性准则;然后;叙述了称为“事务语义协议”的并发存取控制原理和方法,给出了全局调度和交付的算法和方法正确性证明。     

MDBS中一种改进的并发事务调度算法

MDBS中并发事务的调度策略必须满足可串行化准则,论文主要分析以事务提交图为中心的调度算法TM2,虽然保证了全局事务提交顺序的可串行化,但在提交时才进行冲突检测方式存在缺点,提出改进后的事务调度算法TM3不仅保证了全局事务的可串行化和防止了全局死锁的发生,还提高了全局事务执行的并发度。最后通过实验在数据库加速引擎中进行模拟,对比了两种调度算法的性能。     

A theory of global concurrency control in multidatabase systems

This article presents a theoretical basis for global concurrency control to maintain global serializability in multidatabase systems. Three correctness criteria are formulated that utilize the intrinsic characteristics of global transactions to determine the serialization order of global subtransactions at each local site. In particular, two new types of serializability, chain-conflicting serializability and sharing serializability, are proposed and hybrid serializability, which combines these two basic criteria, is discussed. These criteria offer the advantage of imposing no restrictions on local sites other than local serializability while retaining global serializability. The graph testing techniques of the three criteria are provided as guidance for global transaction scheduling. In addition, an optimal property of global transactions for determinating the serialization order of global subtransactions at local sites is formulated. This property defines the upper limit on global serializability in multidatabase systems.     

Locking and Deadlock Detection in Distributed Data Bases

This paper descrbes two protocols for the detection of deadlocks in distributed data bases–a hierarchically organized one and a distributed one. A graph model which depicts the state of execution of all transactions in the system is used by both protocols. A cycle in this graph is a necessary and sufficient condition for a deadlock to exist. Nevertheless, neither protocol requires that the global graph be built and maintained in order for deadlocks to be detected. In the case of the hierarchical protocol, the communications cost can be optimized if the topology of the hierarachy is appropriately chosen.