Transaction Management in Distributed Database System POREL

This paper describes the architecture and implementation algorithm of a transaction managementsuhsystem (TM) in the distributed database system POREL.The TM has been reimplemented on VAX11/750.     

Transaction processing in mobile, heterogeneous database systems

As technological advances are made in software and hardware, the feasibility of accessing information "any time, anywhere" is becoming a reality. Furthermore, the diversity and amount of information available to a given user is increasing at a rapid rate. In a mobile computing environment, a potentially large number of users may simultaneously access the global data; therefore, there is a need to provide a means to allow concurrent management of transactions. Current multidatabase concurrency control schemes do not address the limited bandwidth and frequent disconnection associated with wireless networks. This paper proposes a new hierarchical concurrency control algorithm. The proposed concurrency control algorithm, v-lock, uses global locking tables created with semantic information contained within the hierarchy. The locking tables are used to serialize global transactions, detect and remove global deadlocks. Additionally, data replication, at the mobile unit, is used to limit the effects of the restrictions imposed by a mobile environment. The replicated data provides additional availability in case of a weak connection or disconnection. Current research has concentrated on page and file-based caching or replication schemes to address the availability and consistency issues in a mobile environment. In a mobile, multidatabase environment, local autonomy restrictions prevent the use of a page or file-based data replication scheme. This paper proposes a new data replication scheme to address the limited bandwidth and local autonomy restrictions. Queries and the associated data are cached at the mobile unit as a complete object. Consistency is maintained by using a parity-based invalidation scheme. A simple prefetching scheme is used in conjunction with caching to further improve the effectiveness of the proposed scheme. Finally, a simulator was developed to evaluate the performance of the proposed algorithms. The simulation results are presented and discussed.     

Microcomputers and distributed database management systems

The role of a DDBMS is to provide overall system integrity, appropriate data access, and recovery over the whole network. Many of the supermicros with powerful 16 and 32 bit architectures will soon be running advanced DBMS and DDBMS software, thus, playing an important role as a distributed database on a network. The article discusses this and future trends.     

Multilevel security issues in distributed database management systems

This paper describes the security needs in a distributed processing environment common to many enterprises and discusses the applicability of multilevel secure database management systems in such an environment.     

Four reference architectures for distributed database management systems

Five requirements for a centralized database management system and five additional requirements for a distributed database management system suggest definitions of software processors and schemas. These software processors and schemas can be organized into four reference architectures for distributed database management systems which permit comparisons of their advantages and disadvantages.     

Some notes on authorization and transaction management in distributed database systems

Two topics on distributed database systems, in which security is involved are treated.

• 1.(1)Authorization: the dynamical granting, checking and revocation of access privileges to permit users to share data on the one hand and to retain the possibility to restrict such privileges on the other hand.
• 2.(2)Transaction management: to control the execution of database actions comprising a transaction so that it is atomic and recoverable.

Included for both subjects is a survey of various problems arising in the area. Then algorithms to solve these problems are described. Distributed problems are very often more difficult than their non-distributed equivalents. Attention will be paid to how the algorithms deal with the distributed aspect.     

DYFRAM: dynamic fragmentation and replica management in distributed database systems

In distributed database systems, tables are frequently fragmented and replicated over a number of sites in order to reduce network communication costs. How to fragment, when to replicate and how to allocate the fragments to the sites are challenging problems that has previously been solved either by static fragmentation, replication and allocation, or based on a priori query analysis. Many emerging applications of distributed database systems generate very dynamic workloads with frequent changes in access patterns from different sites. In such contexts, continuous refragmentation and reallocation can significantly improve performance. In this paper we present DYFRAM, a decentralized approach for dynamic table fragmentation and allocation in distributed database systems based on observation of the access patterns of sites to tables. The approach performs fragmentation, replication, and reallocation based on recent access history, aiming at maximizing the number of local accesses compared to accesses from remote sites. We show through simulations and experiments on the DASCOSA distributed database system that the approach significantly reduces communication costs for typical access patterns, thus demonstrating the feasibility of our approach.     

Improving the flexibility for replicated data management in distributed database systems

This paper presents a novel scheme for implementing the flexible replica control protocol[13] in the distributed database systems. The scheme requires less nodes to be locked to perform the read(write) operations. This not only provides better performance, but also provides the system designer extra flexibility to implement the protocol.     

Security design in distributed database systems

This paper deals with data security design in a distributed environment. A goal of the paper is to present a multiphase security design methodology reflecting the current approach to data base design. The aggregated distributed environment is considered, and a multilevel logical security system architecture is examined. For each level the content of the security logical schemata is defined and described. Finally, mapping rules between the logical levels are presented.     

政务网络信息系统的异构分布式数据库研究

在当前政务网络信息系统中,多个部门系统数据库之间存在明显的异构性、独立性和自主性,使不同部门之间的信息交流成为难题。采用局部数据集成技术在网络信息中心构建互连通讯平台,开发部门子系统进行数据查询、接收、核对、确认等方法,能够实现政务网络信息系统异构分布式数据库之间的信息共享。     

A toolkit for the incremental implementation of heterogeneous database management systems

The integration of heterogeneous database environments is a difficult and complex task. The A la carte Framework addresses this complexity by providing a reusable and extensible architecture in which a set of heterogeneous database management systems can be integrated. The goal is to support incremental integration of existing database facilities into heterogeneous, interoperative, distributed systems. The Framework addresses the three main issues in heterogeneous systems in tegration. First, it identifies the problems in integrating heterogeneous systems. Second, it identifies the key interfaces and parameters required for autonomous systems to interoperate correctly. Third, it demonstrates an approach to integrating these interfaces in an extensible and incremental way. The A la carte Framework provides a set of reusable, integrating components which integrate the major functional domains, such as transaction management, that could or should be integrated in heterogeneous systems. It also provides a mechanism for capturing key characteristics of the components and constraints which describe how the components can be mixed and interchanged, thereby helping to reduce the complexity of the integration process. Using this framework, we have implemented an experimental, heterogeneous configuration as part of the object management work in the software engineering research consortium, Arcadia.     

Organizational hurdles to distributed database management systems (DDBMS) adoption

Organizations adopt new technologies, ultimately, to enhance their competitive position and profitability. A distributed database management system (DDBMS) is an example of a technology that may enhance a company's profitability but is also likely to change the way it does business. Specifically, DDBMS adoption may disturb the locus of power, alter the organizational structure, and change the culture of a company. This exploratory study examines the organizational factors that influence the likelihood of DDBMS adoption. It identifies organizational hurdles to adoption, offers an agenda for research validation, and proposes strategies for overcoming the identified hurdles.     

面向电力调度系统的分布式数据库管理系统的设计

针对电力实时信息网的实时性和地理上的分散性给信息交换带来的困难,提出基于分布式数据库体系结构的电力实时数据共享的解决方案。     

Object orientation in heterogeneous distributed computing systems

The basic properties of object orientation and their application to heterogeneous, autonomous, and distributed system to increase interoperability ar examined. It is argued that object-oriented distributed computing is a natural step forward from client-server systems. To support this claim, the differing levels of object-oriented support already found in commercially available distributed systems-in particular, the distributed computing environment of the open software foundation and the Cronus system of Bolt Beranek, Newman (BBN)-are discussed. Emerging object-oriented systems and standards are described, focusing on the convergence toward a least-common-denominator approach to object-oriented distributed computing embodied by the object management group's common object request broker architecture     

Adaptive load sharing in heterogeneous distributed systems

In this paper, we study the performance characteristics of simple load sharing algorithms for heterogeneous distributed systems. We assume that nonnegligible delays are encountered in transferring jobs from one node to another. We analyze the effects of these delays on the performance of two threshold-based algorithms called Forward and Reverse. We formulate queuing theoretic models for each of the algorithms operating in heterogeneous systems under the assumption that the job arrival process at each node in Poisson and the service times and job transfer times are exponentially distributed. The models are solved using the Matrix-Geometric solution technique. These models are used to study the effects of different parameters and algorithm variations on the mean job response time: e.g., the effects of varying the thresholds, the impact of changing the probe limit, the impact of biasing the probing, and the optimal response times over a large range of loads and delays. Wherever relevant, the results of the models are compared with the M/M/ 1 model, representing no load balancing (hereafter referred to as NLB), and the M/M/K model, which is an achievable lower bound (hereafter referred to as LB).     

Analysis of replication in distributed database systems

The authors develop an approximate analytical model to study the tradeoffs of replicating data in a distributed database environment. Several concurrency control protocols are considered, including pessimistic, optimistic, and semi-optimistic protocols. The approximate analysis captures the effect of the protocol on hardware resource contention and data contention. The accuracy of the approximation is validated through detailed simulations. It is found that the benefit of replicating data and the optimal number of replicates are sensitive to the concurrency control protocol. Under the optimistic and semi-optimistic protocols, replications can significantly improve response time with an additional MIPS (million instructions per second) requirement to maintain consistency among the replicates. The optimal degree of replication is further affected by the transaction mix (e.g. the fraction of read-only transactions), the communications delay and overhead, the number of distributed sites, and the available MIPS. Sensitivity analyses have been carried out to examine how the optimal degree of replication changes with respect to these factors     

Dynamic transaction routing in distributed database systems

The authors investigate dynamic transaction routing strategies for locally distributed database systems in which the database is partitioned and distributed among multiple transaction-processing systems, and the incoming transactions are routed by a common front-end processor. If a transaction issues a database request referencing a nonlocal database partition, the request has to be shipped to the system owing the referenced partition for processing. Various dynamic strategies are studied. Their performance is compared with that of the optimal static strategy. A class of dynamic transaction routing strategies which take into account routing history and minimize the estimated response time of incoming transactions is proposed; they are found to provide a substantial improvement over the optimal static strategy. The robustness of the strategies is further studied through sensitivity analysis over various transaction loads, communication overheads, and database reference distributions