数据网格中的数据复制技术研究

数据复制技术是数据网格系统中广泛采用的改善网格系统性能的关键技术之一。与传统分布式系统应用领域中的复制相比，数据网格中的复制技术在复制目标、复制粒度、复制关键技术等方面表现出独特性质。本文将数据复制技术概括为副本创建、数据传输、副本删除、副本选择、副本一致性管理、安全管理等环节，深入分析、探讨了数据网格系统中数据复制的各项关键技术，为建立综合的数据网格复制策略和技术框架提供了全面的技术分析。最后，对数据复制技术下一步的研究方向作了分析和预测。     

基于消息通信的分布式系统最终一致性平台

在分布式系统中为了满足高性能和吞吐量,一般采用异步消息通信方式,但消息通信没有解决分布式事务不一致问题。针对这个问题,提出建立一致性保障平台,通过这个平台实现最终一致性。首先,使系统满足幂等性以及业务数据与消息生产消费记录强一致性;其次,建立消息监控机制,根据监控规则和消费生产消费记录,判定消息正常还是需要补偿操作或者幂等操作,从而保证分布式系统基于消息通信的最终一致;最后,在整个设计实现过程中采用关注点分离和横向切分的思想与工程化的方法,实现一致性保障平台。通过实验和分析证明比较得出,与异步消息通信相比,分布式消息通信性能更优越;一致性保障平台能及时发现不一致并由系统及时处理,实现最终一致,即可以完全保障系统最终一致性;而且该平台通过平台化的实现方式在应用中可以快速复用到数十个业务系统。由此得出一致性保障平台可以解决分布式交易系统事务最终一致性问题,不仅性能优越而且经济。     

An optimized strategy for update path selection in unstructured P2P networks

With the growth of P2P file sharing systems, people are no longer satisfied with the sharing of the read-only and static files, and thus the systems with mutually writable and dynamic files have emerged, resulting in the replica inconsistency problem. To maintain the replica consistency, too many update messages need to be redundantly propagated due to the lack of the sharing of globally updated path information in the existing strategies. To address this problem, we propose an optimized strategy for update path selection, which makes the nodes share the update path information by using clone, variation and crossover operations for the update paths. We also present a repeated update strategy to cope with the churn problem so as to maintain replica consistency as far as possible even if some nodes temporarily leave the network. The simulation results show that our strategy can significantly reduce the number of the redundant update messages without lowering the message coverage, thus improving the availability of the unstructured P2P networks.     

A Distributed Stream Query Optimization Framework through Integrated Planning and Deployment

This paper addresses the problem of optimizing multiple distributed stream queries that are executing simultaneously in distributed data stream systems. We argue that the static query optimization approach of "plan, then deployment” is inadequate for handling distributed queries involving multiple streams and node dynamics faced in distributed data stream systems and applications. Thus, the selection of an optimal execution plan in such dynamic and networked computing systems must consider operator ordering, reuse, network placement, and search space reduction. We propose to use hierarchical network partitions to exploit various opportunities for operator-level reuse while utilizing network characteristics to maintain a manageable search space during query planning and deployment. We develop top-down, bottom-up, and hybrid algorithms for exploiting operator-level reuse through hierarchical network partitions. Formal analysis is presented to establish the bounds on the search space and suboptimality of our algorithms. We have implemented our algorithms in the IFLOW [CHECK END OF SENTENCE] system, an adaptive distributed stream management system. Through simulations and experiments using a prototype deployed on Emulab [CHECK END OF SENTENCE], we demonstrate the effectiveness of our framework and our algorithms.     

A novel clustered MongoDB-based storage system for unstructured data with high availability

More and more unstructured data are produced and consumed over network. How to maintain these data and improve the availability and scalability of the storage systems has become a considerable challenge. Although some NoSQL systems such as Dynamo, Cassandra, MongoDB have provided different advantages for unstructured data management, no one can provide flexible query functions like MongoDB, meanwhile guarantee the availability and scalability as Cassandra simultaneously. This paper presents a new high available distributed storage system called MyStore based on an optimized clustered MongoDB for unstructured data. Consistent hash is used to distribute data on multiple MongoDB nodes by applying virtual node method. NWR mode is applied to provide automatic backup operation and guarantee data consistency. And a gossip protocol is taken for exchanging information of failures in the system. Moreover, a user-friendly interface module and an efficient cache module are designed for improving the usability of the system. Based on above strategies, the system can realize high availability for unstructured data storage, while providing complex query functions like rational databases. Moreover, it is applied in a multi-discipline virtual experiment platform named VeePalms that has run practically. Experimental evaluation shows that the methodology is powerful enough not only to enhance the data availability, but also to improve the server’s scalability.     

Regeneration with virtual copies for distributed computing systems

The authors consider the consistency control problem for replicated data in a distributed computing system (DCS) and propose a new algorithm to dynamically regenerate copies of data objects in response to node failures and network partitioning in the system. The DCS is assumed to have strict consistency constraints for data object copies. The algorithm combines the advantages of voting-based algorithms and regeneration mechanisms to maintain mutual consistency of replicated data objects in the case of node failures and network partitioning. The algorithm extends the feasibility of regeneration to DCS on wide area networks and is able to satisfy user queries as long as there is one current partition in the system. A stochastic availability analysis of the algorithm shows that it provides improved availability as compared to previously proposed dynamic voting algorithms     

Middleware extensions that trade consistency for availability

Replicated distributed object systems are deployed to provide timely and reliable services to actors at distributed locations. This paper treats applications in which data updates are dependent on satisfaction of integrity constraints over multiple objects. Network partitions, caused by occasional link failures, overload or attacks create problems in keeping both consistency and availability in such networks. We propose a means of achieving higher availability by providing partition‐awareness in middleware. The general approach has been illustrated by implementing a number of CORBA extensions that trade consistency for availability during network partitions. This paper contains a thorough experimental evaluation that presents the gains and costs of our approach. The experiments clearly illustrate the benefit of our protocols in terms of significantly higher availability and the number of performed operations. Copyright © 2009 John Wiley & Sons, Ltd.     

CMV: File consistency maintenance through virtual servers in peer-to-peer systems

Peer-to-peer (P2P) has become a mainstream architecture in numerous diverse distributed applications. However current P2P systems do not provide consistency guarantees under multiple reader multiple writer scenarios. Such a feature is desirable as well as necessary for supporting more diverse applications than merely file-sharing systems. In this paper, we develop a highly scalable and efficient algorithm, called Consistency Maintenance through Virtual servers (CMV), in P2P systems. In this algorithm, consistency of each dynamic file is maintained by a Virtual Server (VS). A file update can only be accepted through the VS to ensure one-copy serializability consistency. The VS of a file is a logical network composed of multiple Replica Peers (RPs) that have replicas of the file. Mathematical analysis is performed for optimal parameter selections that achieve minimum overhead messages for maintaining file consistency. Simulation experiments are conducted to compare the performance of the proposed CMV algorithm with two existing schemes, namely the rumor spreading based scheme and the Update Propagation Through Replica Chain (UPTReC) scheme. Our results show that CMV can quickly commit update to the system and significantly reduce (by more than 90%) overhead messages compared to these schemes under various system conditions.     

Binary vote assignment on a grid for efficient access of replicated data

Data replication can be used to improve the availability of data in a distributed database system. In such a system, a mechanism is required to maintain the consistency of the replicated data. Grid structure (GS) technique based on quorum is one of the latest solutions for performing this while providing a high availability of the system. It was shown in the previous study, that it still requires a bigger number of copies be made available to construct a quorum. So it is not suitable for the large systems. In this paper, we propose a technique called the binary vote assignment on grid (BVAG) technique by considering only neighbors have the replicated data. In comparison to the GS technique, BVAG requires lower communication cost for an operation, while providing higher data availability, which is preferred for large systems.     

A fault-tolerant algorithm for replicated data management

We examine the tradeoff between message overhead and data availability that arises in the design of fault-tolerant algorithms for replicated data management in distributed systems. We propose a property called asymptotically high resiliency which is useful for evaluating the fault-tolerance of replica control algorithms and distributed mutual exclusion algorithms. We present a new algorithm for replica control that can be tailored (through a design parameter) to achieve the desired balance between low message overhead and high data availability. Further, we show that for a message overhead of O(√(Nlog N)), our algorithm can achieve asymptotically high resiliency     

胖树中的分布式动态容错路由

面向云计算的超大规模互连网络增加了对网络容错的要求,容错已成为互连网络的重要问题.为了保证网络的高可用性和高性能,文中基于胖树网络拓扑提出了一种分布式的动态容错路由方法.该方法通过引入一套链路失效消息传播机制和一套基于链路失效信息的动态容错路由算法来实现胖树网络的分布式动态容错.相比已有方法,该方法不增加网络硬件和路由路径长度,并且具有高执行效率和高性能.实验结果表明,在m端口交换机构成的胖树中,该方法可以容忍任意m/2-1条失效链路并以高概率容忍更多条失效链路的组合,同时保持网络的高性能.     

Adaptive popularity-driven replica placement in hierarchical data grids

Data grids support access to widely distributed storage for large numbers of users accessing potentially many large files. Efficient access is hindered by the high latency of the Internet. To improve access time, replication at nearby sites may be used. Replication also provides high availability, decreased bandwidth use, enhanced fault tolerance, and improved scalability. Resource availability, network latency, and user requests in a grid environment may vary with time. Any replica placement strategy must be able to adapt to such dynamic behavior. In this paper, we describe a new dynamic replica placement algorithm, Popularity Based Replica Placement (PBRP), for hierarchical data grids which is guided by file “popularity”. Our goal is to place replicas close to clients to reduce data access time while still using network and storage resources efficiently. The effectiveness of PBRP depends on the selection of a threshold value related to file popularity. We also present Adaptive-PBRP (APBRP) that determines this threshold dynamically based on data request arrival rates. We evaluate both algorithms using simulation. Results for a range of data access patterns show that our algorithms can shorten job execution time significantly and reduce bandwidth consumption compared to other dynamic replication methods.     

基于分布式生成树的副本定位服务

副本定位服务(RLS)是Globus的标准组件之一,它为客户提供副本的注册和定位服务。Globus中原始的RLS服务通过静态配置,在多个冗余的副本定位索引(RLI)节点间建立起一个分层的网络,从而确保服务的可靠性和负载平衡。结合P2P网络中的资源定位方法,提出了一种基于分布式生成树(DST)的副本定位服务(DST-RLS)。首先用DST-RLS节点网络替换原有的RLI节点的静态配置,从而提供了一种具有自组织性,错误容忍和更高易扩展能力的副本定位服务。接下来对相关算法进行了描述,并对DST-RLS节点网络的性能进行了分析。最后,通过仿真模型,对DST-RSL性能进行了验证。     

KESS元数据处理一致性协议

在麒麟分布式加密存储系统中,分布式元数据处理在系统发生异常时会出现不一致的情况。为了解决这一问题,提出了元数据处理一致性协议2PC-MP。该协议引入事务序号,保证日志记录和消息交互的一致性;增加悬挂队列,避免参与者进程因网络异常而阻塞;增加回退队列,解决用户登录session失效后无法回退的问题;通过分布式日志保证系统故障后的快速恢复。结果表明,2PC-MP协议能够保证元数据处理的一致性和提高系统的性能。     

基于知识的网络管理模块的扩展

智能网络系统管理是一个多agent的系统。传统的网络管理平台和体系结构主要基于面向对象的网络信息管理,其具体的实现主要由标准化组织如ISO、ITU-T和IETF规范。知识共享研究组织(KSE)主要定义了基于知识的agen的内部和外部的行为,而TINA-C集中研究分布处理环境下的信息网络。该文试图将知识系统扩展到网络资源对象中并建立新的信息模型来支持复杂的网络管理作业所需的智能操作。该文的思想将有助于实际系统的进一步研究和实现     

Optimal cost design of replicated data in distributed database systems

Replicated data are a common strategy for achieving ultra-availability in fault-tolerant distributed database systems. Replication, however, requires additional resources, for example additional costs in terms of maintenance effort, software complexity, hardware requirements and time needed for testing consistency. Various replication control protocols have been developed to ensure data consistency. In this paper, we develop analytical methods for the quorum-consensus replication protocol that minimizes the total system cost by determining the optimal read quorum, the optimal number of system sites, or the optimal site availability assuming that (1) the cost of an individual site is an increasing function of the availability of the site and (2) the two system unavailable modes (i.e. read and write) can have different penalty costs. Several numerical examples and applications are provided to illustrate the results.     

The Globus Replica Location Service: Design and Experience

Distributed computing systems employ replication to improve overall system robustness, scalability, and performance. A Replica Location Service (RLS) offers a mechanism to maintain and provide information about physical locations of replicas. This paper defines a design framework for RLSs that supports a variety of deployment options. We describe the RLS implementation that is distributed with the Globus Toolkit and is in production use in several Grid deployments. Features of our modular implementation include the use of soft-state protocols to populate a distributed index and Bloom filter compression to reduce overheads for distribution of index information. Our performance evaluation demonstrates that the RLS implementation scales well for individual servers with millions of entries and up to 100 clients. We describe the characteristics of existing RLS deployments and discuss how RLS has been integrated with higher-level data management services.     

基于纠删编码P2P存储系统的数据维护算法

纠删编码是一种通过产生数据冗余来提高P2P存储系统容错性和可用性的有效方法。对基于纠删编码的非结构化P2P存储系统来说,使用洪泛算法进行数据维护会在网络中产生大量冗余消息,系统效率低。本文提出一种使用二叉树来保存文件块的信息的算法。二叉树结构被建立以后,更新消息在二叉树中的节点间传播,不需要在网络中洪泛。分析表明,相较于洪泛算法,本算法有效减少了冗余的消息数量,提高了数据维护的效率,而付出的是极小的存储代价。     

网格环境中基于对等代理的复制定位机制

针对目前网格环境中复制定位机制存在的问题，并综合考虑其中的优势，提出了一种基于对等代理的分布式复制定位机制，即通过对等代理组成分布式的虚拟全系统，实现全域网络里最大有效范围内的复制定位，比起传统的基于分级机制和分布式查询与索引机制的定位服务，在性能、安全、可靠性和有效性上都有很大的改善和提高。     

云计算架构下的动态副本管理策略

为有效管理云计算环境中的数据副本,减少系统带宽消耗、最小化响应时间和平衡负载,提出了一种动态副本管理策略.通过建立文件可用性和副本数量间的关系模型来计算系统应维持的最小副本数量;基于数据被请求访问的次数和传输花费进行副本放置;数据被请求时结合节点间带宽和节点效用选择副本.实验结果表明了该策略的正确性有效性.