Verification of Knowledge-Based Systems Using Predicate/Transition Nets

As expert-system technology gains broader acceptance, the need to build and maintain large-scale knowledge-based systems (KBSs) will assume greater importance. Traditional approaches to KBS verification generally contain no predicate/transition (PrT) net models, thus making them slow for the large-scale KBS with chained errors. This paper proposes an attractive alternative to KBS verification, in which the KBS is modeled as a PrT-net model. Then, the least fixpoint semantics of the PrT-net model can be introduced into the KBS for the purpose of speeding up the computations of the KBSs. The significance of this paper is that seven propositions are formulated to detect errors of redundancy, subsumption, unnecessary condition, circularity, inconsistency, dead end, and unreachable goal. Thus, the performance of a computer-aided-design tool for KBSs can be improved to some extent. Meanwhile, specification languages, including Programming in Logic, Frame-and-Rule-Oriented Requirements Specification Language, and the like, are suitable to this approach.     

An Information-Based Model for Failure-Handling in Distributed Database Systems

We consider the failure atomicity problem of distributed transactions in conjunction with the maximization of database availability. We propose a new information-based model for the distributed transaction-execution, which explicitly expresses the information at each stage during a protocol. In addition to rederiving certain existing results, we prove a fundamental relation among the site failures and the network partitioning. We propose a realistic model for site failures under which we show that the costs of commit and termination protocols can be greatly reduced. Finally, we explore the possible recovery strategies for a failed site and show how they are improved under our site failure model.     

分布式多媒体数据库系统

着重分析分布式多媒体数据库系统的特有问题,包括实时性、时空同步以及服务质量的描述与实现等.给出了解决这些问题的策略和方法.研究了各种不同的实现途径——远程调用、中间件和Agent构造的方式;指出了这些实现方法的优点及其不足.     

Five Steps to Construct a Model of Data Allocation for Distributed Database Systems

The allocation of data to nodes in a computer communications network is a critical issue in distributed database design, specially in wide area network where optimizing the distributed queries is the main objective. Numerous models of data allocation have been presented so far (Ceri and Pelagatti, 1986; Lee et al., 1994; Apers, 1988; Mahmood et al., 1994; Theel, 1994; Saha and Mukherjee, 1994; Walfson and Jajodia, 1995), but no optimal or universally accepted algorithm exists yet, and very few algorithms have been implemented to date (Rob and Cornonel, 1993). In this paper a method to construct a model of data allocation for distributed database system has been proposed. The adopted method for the construction of the data allocation model leads to an efficient model of its kind. The proposed model not only solves distributed database design problem, but also solves distributed database maintenance problem by covering initial allocation and post allocation of the data. The efficiency of proposed model has been proved by comparing proposed model with other relevant models.     

分布式数据库中一致性与可用性的关系

随着各类应用在数据量和业务量上的扩展,单机数据库系统越发难以应对现实需求。分布式数据库能够根据业务的需求动态地扩容,因此逐步开始受到应用的青睐。近年来,分布式数据库产品层出不穷,并在互联网应用中被大量投入使用。然而,分布式数据库的系统复杂度前所未有。为了让系统可用,设计者需要在多种属性中作合理选择和折中。这造成现有的数据库产品形态各异、优缺点对比分明。至今为止,尚未有人对分布式数据库的设计空间和折中方案进行过深入分析和整理。本文作者在对多个分布式数据库产品进行深入理解之后认识到：分布式数据库系统的设计方案可以通过三个属性进行基本刻画–操作一致性、事务一致性和系统可用性。虽然这三个属性并不新颖,但它们在数据库语境下的含义在文献中尚未得到充分澄清。本文对这三个属性进行澄清,并通过它们对典型数据库产品的格局进行概括、对现有的分布式数据库技术进行综述。此外,本文还对这三个属性之间的相互关系进行深入分析,以期帮助未来的开发者在分布式数据库的设计过程中作出合理选择。     

浅谈分布式数据库系统查询优化

分布式数据库系统的查询优化,就是要寻找执行代价最小的查询执行策略,使系统执行效率达到最高。我们在应用中需要选择适当优化方法,在执行代价和便捷度之间得到最佳执行方案。     

A Model for Query Decomposition and Answer Construction in Heterogeneous Distributed Database Systems

The problem of retrieving information from a collection of heterogeneous distributed databases has attracted a number of solutions. However, the task of integrating established database systems is complicated not only by the differences between the database systems themselves, but also by the differences in structure and semantics of the information contained within them. The problem is exacerbated when one needs to provide access to such a system for naive end-users.This paper is concerned with a Knowledge-Based Systems approach to solving this problem for clearly bounded situations, in which both the domain and the types of query are constrained. At the user interface, dialogue is conducted in terms of concepts with which the user is familiar, and these are then mapped into appropriate database queries. To achieve this a model for query decomposition and answer construction has been used. This model is based around the development of an Intensional Structure containing information necessary for the recapture of semantic information lost in the query decomposition process and required in the answer construction process. The model has been successfully implemented in combination with an embedded KBS, within a five-layer representation model.     

利用谓词/变迁网证明的一阶谓词逻辑命题

研究了证明一般的一阶谓词逻辑命题的方法,根据网逻辑的思想,利用谓词/变迁网对一般形式的一阶谓词逻辑命题进行了图形表示,提出了2种一阶谓词逻辑命题的证明方法：图形证明法和矩阵证明法。举出一个实际的例子来说明证明思路。     

分布式数据库系统的复制机制及应用

分布式数据库系统是比较成熟的数据库技术，在实际中得到了广泛的应用，本文介绍了分布式数据库系统复制机制的核心技术，分析了各种复制技术的优缺点，最后确定了一个完整的复制方案，并应用到实际中。     

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。     

Protocols for Deadlock Detection in Distributed Database Systems

In distributed databases, deadlocks may occur due to conflicts in data file lockings A system is in a deadlock if and only if there is a directed cycle in its demand graph. However, due to the inherent communication delay in a distributed system, it is not easy to construct a consistent demand graph for a distributed system. In this paper, three deadlock detection protocols are discussed. The first protocol uses two communication phases. The second protocol uses a single communication phase. Based on the second protocol, a one-phase hierarchical deadlock detection protocol is developed.     

分布式系统可靠性模型

文章首先给出分布式系统的一般描述;然后采用建立在图论、概率论及布尔代数基础上的网络分析法建立了分布式系统的可靠性模型,用以综合考虑系统拓朴结构、任务集、通讯路径集和处理单元集等对系统可靠性的影响;最后,针对二维TORUS网,给出了建模实例以及模型的有关应用。     

基于分布式应用技术的分布式数据库接力棒访问模型

首先介绍了分布式应用技术及分布式数据库特点，然后提出了访问分布式数据库的接力棒访问模型。该模型将多层体系结构的分布式应用技术应用于分布式数据库的访问上，将请求异地数据和异地数据请求都以消息机制进行推送，从而解决在访问分布式数据库情况下网络开销过大、数据库互连复杂、数据更新时保证一致性困难等问题。     

分布式数据库通用查询对象模型

提出基于CORBA的分布式数据库通用查询对象模型，该模型结合目前最先进的分布式对象技术，异构数据库访问技术以及为了解决异构分布式计算机系统数据传输问题而提出了的数据类型鉴别器技术，为异构分布式数据库系统提供可跨平台移植的，跨数据库查询的具有统一接口的分布式数据库通用查询构件，利用分布式数据库通用查询对象模型概念的实现，成功开发出南方电力调度中心异构分布式计算机信息交换系统软件，给出了使用该软件在分布式异构系统中实现数据通用查询的实验结果，证明是可行的。     

基于分布式数据库的挖掘模型

给出了一种基于分布式数据库的挖掘模型及其相应的一种有效的挖掘算法,该模型和算法在减轻网络频繁的通讯负担、体现并行计算以及异构数据挖掘方面有其独特的优点.