Semantic-based locking technique in object-oriented databases

This paper presents a locking-based concurrency control dealing with three important issues in object-oriented databases: semantics of methods, nested method invocation and referentially shared object. In the proposed scheme, locks are required for the execution of methods instead of atomic operations. This can reduce the locking overhead and deadlocks due to lock escalation. Also, a way of automating commutativity of methods is provided. In addition, concurrency is increased further by use of run-time information. A performance study is conducted by means of simulation using 007 benchmark. Through the simulation, the proposed technique is then compared with the two existing techniques. The performance studies show that the proposed scheme is superior to existing works.     

A multi-granularity locking model for concurrency control inobject-oriented database systems

A locking model adopting a multi-granularity approach is proposed for concurrency control in object-oriented database systems. The model is motivated by a desire to provide high concurrency and low locking overhead in accessing objects. Locking in schemas and locking in instances are developed separately and then are integrated. Schema changes and composite objects are also taken into account. A dual queue scheme for efficient scheduling of lock requests is developed. The model consists of a rich set of lock modes, a compatibility matrix, and a locking protocol. Characteristic query examples on single class, class lattice, and composite objects are used to illustrate the comparison between the ORION model and the proposed model. It is shown that our locking model has indeed made some improvements and is suitable for concurrency control in object-oriented databases     

一种基于时间戳的面向对象数据库的并发控制算法

本文提出了一种基于时间戳的面向对象数据库（ＯＯＤＢ，ｏｂｊｅｃｔ－ｏｒｉｅｎｔｅｄｄａｔａｂａｓｅ）的并发控制算法．设计这一算法时，我们首先按照ＯＯＤＢ的数据模型对传统的时间戳算法进行扩展，然后利用抽象数据类型的语义定义了相容性矩阵，对不同级别的并发性冲突操作进行不同的处理，从而达到在系统开销尽可能小的情况下，尽可能大地增加数据库存取操作的并发度的效果．这一算法的优点是：①利用对象版本的时间信息减少了数据项的读写时间戳所需的额外存储空间；②消除了封锁算法带来的封锁及死销预防或死锁检测所需要的系统开销。本文还用模拟的方法给出了新算法和ＯＲＩＯＮ中使用的封锁算法之间的性能比较     

A transaction model and multiversion concurrency control for mobile database systems

Transaction management on Mobile Database Systems (MDS) has to cope with a number of constraints such as limited bandwidth, low processing power, unreliable communication, and mobility etc. As a result of these constraints, traditional concurrency control mechanisms are unable to manage transactional activities to maintain availability. Innovative transaction execution schemes and concurrency control mechanisms are therefore required to exploit the full potential of MDS. In this paper, we report our investigation on a multi-versions transaction processing approach and a deadlock-free concurrency control mechanism based on multiversion two-phase locking scheme integrated with a timestamp approach. We study the behavior of the proposed model with a simulation study in a MDS environment. We have compared our schemes using a reference model to argue that such a performance comparison helps to show the superiority of our model over others. Experimental results demonstrate that our model provide significantly higher throughput by improving degree of concurrency, by reducing transaction wait time, and by minimizing restarts and aborts.     

Object-based semantic real-time concurrency control with boundedimprecision

The paper describes a concurrency control technique for real-time object-oriented databases that supports logical consistency and temporal consistency, as well as bounded imprecision that results from their trade-offs. The concurrency control technique uses a semantic locking mechanism within each object and user-defined conditional compatibility over the methods of the object. The semantics can specify when to sacrifice precise logical consistency to meet temporal consistency requirements. It can also specify accumulation and bounding of any resulting logical imprecision. The authors show that this technique, under certain general restrictions, can preserve global correctness and bound imprecision by proving it can guarantee a form of epsilon serializability specialized for object-oriented databases     

A multigranularity locking model for RDF

The advent of Linked Data is spurring the deployment of applications that use the RDF data model at the information tier. In addition to querying RDF data, there is also the requirement for online updates with suitable concurrency control. Client sessions in Web applications are organized as transactions involving requests that read and write shared data. Executing concurrently, these sessions may invalidate each other’s data. This paper presents a locking model, which is a variant of multigranularity locking protocol (MGL), to address isolation between transactions that manipulate RDF data. Four “hierarchically” related granules are defined, as well as new read/write operations and their corresponding lock modes, specifically created for the RDF data model. These new operations allow greater concurrency than the classical read/write operations in relational databases. We assessed the performance of the proposed locking model through model simulation.     

Performance analysis of concurrency control using locking withdeferred blocking

The concurrency control (CC) method employed can be critical to the performance of transaction processing systems. Conventional locking suffers from the blocking phenomenon, where waiting transactions continue to hold locks and block other transactions from progressing. In a high data contention environment, as an increasing number of transactions wait, a larger number of lock requests get blocked and fewer lock requests can get through. The proposed scheme reduces the blocking probability by deferring the blocking behavior of transactions to the later stages of their execution. By properly balancing the blocking and abort effects, the proposed scheme can lead to better performance than either the conventional locking or the optimistic concurrency control (OCC) schemes at all data and resource contention levels. We consider both static and dynamic approaches to determine when to switch from the nonblocking phase to the blocking phase. An analytical model is developed to estimate the performance of this scheme and determine the optimal operating or switching point. The accuracy of the analytic model is validated through a detailed simulation     

Single-site and distributed optimistic protocols for concurrencycontrol

The authors consider that, in spite of their advantage in removing the overhead of lock maintenance and deadlock handling, optimistic concurrency control methods have been applied less in practice than locking schemes. Two complementary approaches are introduced that may help render the optimistic approach practically viable. For the high-level approach, integration schemes can be utilized so that the database management system is provided with a variety of synchronization methods each of which can be applied to the appropriate class of transactions. The low-level approach seeks to increase the concurrency of the original optimistic method and improve its performance. The author examines the low-level approach in depth, and presents algorithms that aim at reducing back-ups and improve throughput. Both the single-site and distributed networks are considered. Optimistic schemes using time-stamps for fully duplicated and partially duplicated database networks are presented, with emphasis on performance enhancement and on reducing the overall cost of implementation     

An Object-Oriented Transaction Model

In object-oriented database systems(OOBSs),the traditional transaction models are no longer suitable because of the difference between the object-oriented data model(OODM)and the conventional data models(e.g.relational data model).In this paper,transction models for advanced database applications are reviewed and their shortcomings are analyzed.Exchangeability of operations is proposed instead of commuativity and recoverability for using more semantics in transaction management.As a result,an object-oriented transaction model(in short,OOTM)is presented.It is not modeled for some special application,but directly based on object-oriented paradigms.A transaction is regarded as an interpretation of a metho.Each transaction(even subtransactions)keeps relative ACID(Atomicity,Consistency,Isolation,Durability)properties,therefore the special problems appearing in OOBSs such as“long transactions”,“visibility of inconsistent database state”can be solved.     

移动实时嵌套事务的并发控制

在移动计算环境中，事务移动性和无线网络固有的缺陷使得传统分布式实时事务管理机制不足以支持移动实时事务的执行，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．该文着重研究移动实时事务的并发控制机制．首先，该文给出了一个考虑事务定时限制以及移动性的嵌套事务模型．然后，为减少移动分布式环境中解决数据冲突的开销，该文研究了一种结合优先级夭折和优先级继承的基于封锁的并发控制协议PAI-2PL．当高优先级事务被低优先级事务阻塞时，对于相同家族事务，采用优先级继承方法解决冲突；而对于不同家族事务，则夭折重启低优先级事务．另外，为减少由于断接所引起的无效阻塞，PAI-2PL允许低优先级事务夭折处于断接状态的高优先级事务．通过性能测试，表明所提出的事务模型及并发控制机制能提高实时事务的成功率．     

Restricted admission control in view-oriented transactional memory

This paper proposes a Restricted Admission Control (RAC) scheme for View-Oriented Transactional Memory. The scheme can control the number of threads concurrently accessing a view in order to reduce the number of aborts of transactions. The RAC scheme has the merits of both the locking mechanism and the transactional memory. A theoretical model is proposed to analyze the performance of the RAC scheme and to provide guidance for dynamic adjustment of the number of concurrent threads accessing the same view. Experimental results demonstrate that theoretical RAC model can mostly provide correct guidance to transactional concurrency control. Our RAC implementation shows that RAC can optimize concurrency control of transactions and performs much better than conventional transactional memory systems such as TinySTM that have no dynamic admission control.     

Analysis of hybrid concurrency control schemes for a high datacontention environment

Analytical models are developed to study hybrid CC (concurrency control) schemes which employ a different CC scheme to handle rerun transactions, since their characteristics are different from the first run of transactions. These include switching to static or dynamic locking during rerun (referred to as static and dynamic hybrid OCC (optimistic concurrency control) schemes, respectively), and switching to broadcast OCC during rerun, while doing pure OCC for the first run. In a high data contention environment where locking is inferior to OCC, analysis shows that the performance can be substantially improved by using this hybrid approach and the authors study the tradeoff of the different hybrid CC schemes. The analytic models are based on a decomposition approach and use a mean-value-type analysis. The accuracy of the analysis is validated through simulations     

基于DHP-2PL的分布式实时数据库的并发控制模型

介绍了几种用于RTDBS(实时数据库系统)中的并发控制模型以及它们在分布式环境下(DRTDBS)的拓展。根据DHP-2PL提出一种改进的并发控制模型。在该模型中，引入优先级分配机制和加锁检测机制。通过这两种机制可更好地协调DRTDBS中一致性与截止期的关系，同时也解决了DHP-2PL中事务高重启率的问题。     

A nonrestrictive concurrency control protocol for object-oriented databases

We propose an algorithm for executing transactions in object-oriented databases. The object-oriented database model generalizes the classical model of database concurrency control by permitting accesses toclass andinstance objects, by permittingarbitrary operations on objects as opposed to traditional read and write operations, and by allowingnested execution of transactions on objects. In this paper, we first develop a uniform methodology for treating both classes and instances. We then develop a two-phase locking protocol with a new relationship between locks calledordered sharing for an object-oriented database. Ordered sharing does not restrict the execution of conflicting operations. Finally, we extend the protocol to handle objects that execute methods on other objects thus resulting in the nested execution of transactions. The resulting protocol permits more concurrency than other known locking-based protocols.     

The Efficacy of Commutativity-Based Semantic Locking in a Real-World Application

While the dominant approach to persistent storage in practice is to use a relational DBMS, there are some specialist applications that rely on object stores. The performance of these applications depends on the efficiency of the object store's concurrency control mechanism. Today's predominant concurrency control mechanism is strict two-phase object locking. In the 1980s, an interesting alternative was developed: commutativity-based semantic locking. In theory, it can outperform traditional locking schemes in certain scenarios with appropriate commutativity potential. In this paper, we study the real-world performance of different locking strategies in a particular industrial application from the telecommunications sector. We compare object-based locking and commutativity-based semantic locking. We found that, in this application, semantic locking performs equally to, but no better than, object locking in reasonable mixes of real- world transactions, and that it only outperforms in a deliberately contrived mix.     

Synchronization and recovery in a client-server storage system

Client-server object-oriented database management systems differ significantly from traditional centralized systems in terms of their architecture and the applications they target. In this paper, we present the client-server architecture of the EOS storage manager and we describe the concurrency control and recovery mechanisms it employs. EOS offers a semi-optimistic locking scheme based on the multi-granularity two-version two-phase locking protocol. Under this scheme, multiple concurrent readers are allowed to access a data item while it is being updated by a single writer. Recovery is based on write-ahead redo-only logging. Log records are generated at the clients and they are shipped to the server during normal execution and at transaction commit. Transaction rollback is fast because there are no updates that have to be undone, and recovery from system crashes requires only one scan of the log for installing the changes made by transactions that committed before the crash. We also present a preliminary performance evaluation of the implementation of the above mechanisms. Edited by R. King. Received July 1993 / Accepted May 1996     

空间数据库系统中树的并发控制设计与实现

R树是实现快速空间数据处理的重要索引结构之一,但由于其并发控制的复杂性,虽研究已久但仍然很少真正地集成到商用数据库中。树是为了实现并发控制而提出的一种树结构的变形,但它仍然存在幻像等问题。文章分析了树中存在的这一问题并通过设R-linkRR-link计一个基于内存的操作控制列表来预先避免可能造成幻像的并发操作,从而实现完全的并发控制。实验证明所(Operation Control List, OCList)提方案是正确的、低开支的,而且有利于提高系统性能。     

一种基于加锁粒度的分布式高优先级两段锁的并发控制模型

本文通过分析并发控制中的几种锁模型及其在分布式环境下的拓展，提出了一种基于加锁粒度的分布式高优先级两段锁模型。在该模型中，引入一种加锁检测机制，通过该机制可以确定哪些冲突事务可以通过降低加锁粒度来解决冲突。同时提出了该模型的算法。该算法在避免死锁、提高事务的并发度上有很大的改善。     

利用Java即时编译器自动外提循环中的同步操作

同步开销是影响并行程序性能的一个重要方面,如果同步操作出现在循环中,将会使这种影响进一步扩大.为了降低循环中同步操作的开销,本文提出一种利用即时编译器外提Java程序中循环内同步操作的优化算法,并在实际的Java虚拟机中实现.该算法在保证程序语义不变的前提下,大量减少运行时实际执行的同步操作数量,降低同步开销,并能保证外提变换后同步代码块不会太大而降低程序的并发度.实验结果表明该算法能提高程序的整体性能,并且不降低程序的可扩放性.     

Concurrency control in database systems

Ideas that are used in the design, development, and performance of concurrency control mechanisms have been summarized. The locking, time-stamp, optimistic-based mechanisms are included. The ideas of validation in the optimistic approach are presented in some detail. The degree of concurrency and classes of serializability for various algorithms have been presented. Questions that relate arrival rate of transactions with degree of concurrency and performance have been briefly presented. Finally, several useful ideas for increasing concurrency have been summarized. They include flexible transactions, adaptability, prewrites, multidimensional time stamps, and relaxation of two-phase locking