Utility servers in charlotte

Most distributed operating systems are built with a kernel replicated in each machine that supports only basic interprocess communication (IPC) and process control. All other system services, such as memory management, file system, and name service, are distributed in a set of utility servers, which are ordinary processes (except perhaps for some privileges) residing at various machines. Design and implementation of such utility servers in distributed environments are far different from those in a centralized system. This paper presents our experience in building utility servers in Charlotte, a message-based distributed operating system running on a loosely-coupled multicomputer. Utility services in Charlotte are provided by server squads. Each member in a squad covers services to its own community. The squad as a whole co-operatively provides services to the entire system. These servers are designed with the goals of simplicity, efficiency and robustness. They are intended to support a multiprogramming system for the development of distributed algorithms and other distributed applications. We address several major issues in developing a utility server, including the server structure, the management of message buffers, deadlock, and the robustness of server processes. Several utility servers in the Charlotte system are discussed as real examples.     

Distributed cooperative Web servers

Traditional techniques for a distributed web server design rely on manipulation of central resources, such as routers or DNS services, to distribute requests designated for a single IP address to multiple web servers. The goal of the distributed cooperative Web server (DCWS) system development is to explore application-level techniques for distributing web content. We achieve this by dynamically manipulating the hyperlinks stored within the web documents themselves. The DCWS system effectively eliminates the bottleneck of centralized resources, while balancing the load among distributed web servers. DCWS servers may be located in different networks, or even different continents and still balance load effectively. DCWS system design is fully compatible with existing HTTP protocol semantics and existing web client software products.     

基于概率性能感知演化博弈策略的“云+边”混合环境中任务卸载方法

针对“中心云服务器+多个边缘服务器”构成的“云+边”混合环境中多任务卸载效率不足的问题,提出了一种基于概率性能感知演化博弈策略的任务卸载方法。首先,在一个“中心云服务器+多个边缘服务器”构成的“云+边”混合环境中,假设其中分布的边缘服务器具有时变波动的性能,采用一种基于概率性能感知演化博弈策略的任务卸载方法对边缘云服务器的历史性能数据进行概率分析,以获得演化博弈模型;然后,生成服务卸载的演化稳定策略（ESS）,使每个用户都能在获得高满意度的前提下进行任务的卸载。基于云边缘资源位置数据集和云服务性能测试数据集进行模拟实验,在24个连续时间窗口上进行不同方法的测试比较。实验结果表明,所提方法在多个性能指标上都优于传统的贪婪（Greedy）算法、遗传算法（GA）和基于纳什均衡的博弈论算法等任务卸载方法。该方法的平均用户期望达成度相较于三个对比方法分别提升了13.7%、117.0%、13.8%,平均卸载时延分别降低了6.5%、24.9%、8.3%,平均货币成本分别降低了67.9%、88.7%、18.0%。     

Energy-Efficient Redundant Execution of Processes in a Fault-Tolerant Cluster of Servers

This paper investigates into fault tolerance of cluster of servers and their energy efficiency to realize a reliable and energy aware server cluster system. A client issues a request to one server in a server cluster and the server sends a reply to the client in information systems. Once the server stops by fault, the client does not receive a reply of the request. Even if the request is performed on another server on detection of fault of the server, some QoS requirements like response time may not be satisfied. Hence, each request has to be redundantly performed on multiple servers to be tolerant of server faults. The redundant power consumption laxity-based (RPCLB) algorithm is discussed where multiple servers are selected to redundantly and energy-efficiently perform a request process in our previous studies. Since each application process is redundantly performed on more than one server, the larger amount of electric power is consumed. In this paper, we propose a novel and improved RPCLB (IRPCLB) algorithm to reduce the power consumption of servers, where once a process successfully terminates on one server, meaningless redundant processes are forced to terminate on the other servers. In the evaluation, we show the total power consumption of servers and total execution time of processes are reduced in homogeneous and heterogeneous types of clusters by the IRPCLB algorithm than the RPCLB and RR algorithms.     

3-Tiers模式的办公自动化系统WUOA的设计与实现

为保证系统提供更好的客户服务、更快的响应,采用3-Tiers模型作为WUOA的软件体系结构;同时,用组件技术设计实现了WUOA中间层的全部软件功能。但3-Tiers结构中,组件在应用服务器与客户机之间的频繁信息传送,必然导致网络瓶颈问题。笔者提出了“虚拟”应用服务器的概念,设计了适用于OA的虚拟应用服务器VOA-APP,根据组件的使用频度、特征,适度地将组件分布在网络节点和客户机上,在组件分布的基础上,VOA-APP完成组件的实时发布与更新,较好地解决了组件集中与分散分布的协调问题。     

A Review of “What They Don’t Teach You in Library School”

SUMMARY

Distance learning students may not think of the “campus” library as the first place to fulfill their information needs and may not even be aware of the services available to them. One way to reach these students is to adopt and adapt marketing techniques from the business world. This article examines the findings of a survey conducted at Emporia State University concerning the awareness of distance learning services. It will also examine marketing techniques and illustrate how they can be applied to increase awareness of reference support services for distance learners.     

DCFS2的元数据一致性策略

随着集群应用对机群文件系统的性能、容量和规模等需求的日益增长,采用多元数据服务器是机群文件系统发展的必然趋势．基于多元数据服务器的分布式元数据处理是文件系统研究的一个重要问题．机群文件系统DCFS2采用分布式日志技术和改进的两阶段提交协议解决了分布式元数据处理下元数据的一致性问题．性能测试结果表明,DCFS2所采用的基于分布式日志的元数据处理策略能够提供高的I／O性能,并能够保证在元数据服务器失效后文件系统快速恢复．     

网络实验室智能DNS解析系统研究与开发

对智能域名DNS解析设备（广域网负载均衡设备）的开发,旨在解决天津师范大学WEB服务器访问速度慢,打不开网页,加快网络中心访问速度等问题,随着网络应用的不断深入,远程教学和在线辅导已经成为当今继续教育和高校办学的一种手段。因此,本课题研究主要针对智能DNS开发与应用来解决在校园网中网络教学中心进行实时交流,网络中心服务器必须具备提供大量并发访问服务的能力,而网络中心服务器的处理能力和I／O能力就会成为提供服务的瓶颈的问题。     

MOWS: distributed Web and cache server in Java

In this paper, a distributed Web and cache server called MOWS is described. MOWS is written in Java and built from modules that can be loaded locally or remotely. These modules implement various features of Web and cache servers and enable MOWS to run as a cluster of distributed Web servers. In addition to its distributed nature, MOWS can integrate external services using its own external interface. Java programs conforming to this interface can be loaded locally or remotely and executed at the server. The resulting system will potentially provide effective Web access by both utilizing commonly available computing resources and offering distributed server functionality. Design considerations and the system architecture of MOWS are described and several applications of MOWS are described to show the benefits of MOWS.     

基于Nginx负载均衡的动态改进算法

随着过去几十年互联网服务的指数增长,各大网站的访问量急剧上升。海量的用户请求使得热门网站的网络请求率可能在几秒钟内大规模增加。一旦服务器承受不住这样的高并发请求,由此带来的网络拥塞和延迟会极大地影响用户体验。负载均衡是高可用网络基础架构的关键组件,通过在后端引入一个负载均衡器,将工作负载分布到多个服务器来缓解海量并发请求对服务器造成的巨大压力,提高后端服务器和数据库的性能以及可靠性。而Nginx作为一款高性能的HTTP和反向代理服务器,正越来越多地应用到实践中。文中将分析Nginx服务器负载均衡的体系架构,研究默认的加权轮询算法,并提出一种改进后的动态负载均衡算法,实时收集负载信息,重新计算并分配权值。通过实验测试,对比不同算法下的负载均衡性能,改进后的算法能有效提高服务器集群的性能。     

基于RSA的多Web系统智能认证代理模型

随着Web服务的不断深入研究,针对基于多Web服务器环境下的统一认证方法的研究日益重要。论文根据不同Web服务器的认证方式的特点,提出了多Web站点的智能认证代理模型实现统一认证。该模型利用Cookies、监听网络信息、RSA对称密钥体制等关键技术实现单个Web服务器的认证,并采用映射机制建立多Web服务器间的协作。该模型在某高校校园网内得以应用,用户只需登录一次即可完成一个复杂业务,对用户实现了透明认证。     

Distributed Routing of Ads and Bids through Random Walks in the IDOS System

We propose a novel highly distributed system called IDOS (Internet Distributed Objects System) for Buy/Sell interactions on the Internet. In IDOS there is no main or single site that handles user interactions. Instead, each new user “joins” a dynamic graph of users that are currently interested in a specific type of Buy/Sell interaction. The objects for the Buy/Sell interactions are moved between the nodes (users) of the dynamic graph. The Buy/Sell interactions are therefore realized by updating the objects whenever they pass through the current node. This should solve the communication bottleneck caused by the common use of central servers for interactions on the Internet. It does, however, pose a distributed routing problem where messages sent by the users must be distributed between all the “interested” users that are currently connected to the graph. The proposed routing algorithm should overcome the dynamic changes that occur in the graph's topology because some of the users leave the graph while others join it. The proposed solution is to use random walks to distribute the messages, so that the ever-changing topology of the dynamic graph can be overlooked. Consequently, there is no notion of addresses in the system; each user knows only how to communicate with her neighbors. The user must wait until the desired object reaches its node. This simplifies the communication layer, since the centralized client/server TCP/IP infrastructure can be replaced by a point-to-point communication system. We use a system of multiple random “postmen” to overcome the potential communications delay caused by the theoretical large cover-time required by random-walks in graphs. Experimental results of the system as well as how it can be employed to realize public auctions are discussed.     

Faster Web page allocation with neural networks

To maintain quality of service, some heavily trafficked Web sites use multiple servers, which share information through a shared file system or data space. The Andrews file system (AFS) and distributed file system (DFS), for example, can facilitate this sharing. In other sites, each server might have its own independent file system. Although scheduling algorithms for traditional distributed systems do not address the special needs of Web server clusters well, a significant evolution in the computational approach to artificial intelligence and cognitive engineering shows promise for Web request scheduling. Not only is this transformation - from discrete symbolic reasoning to massively parallel and connectionist neural modeling - of compelling scientific interest, but also of considerable practical value. Our novel application of connectionist neural modeling to map Web page requests to Web server caches maximizes hit ratio while load balancing among caches. In particular, we have developed a new learning algorithm for fast Web page allocation on a server using the self-organizing properties of the neural network (NN).     

QoS-based evaluation of file systems and distributed system services for continuous media provisioning

This paper presents a QoS-based performance analysis of file systems and distributed object services for Continuous Media (CM) provisioning, as well as the details and implementation experiences of a continuous media file system and associated CM servers. For this we have implemented QoS-driven CM servers and the Presto continuous media file system (PFS) in the context of a distributed multimedia application development environment, and validate the performance of PFS against that of the conventional Unix file system through an experimental evaluation. Using our CM server prototype, we next examine the effect of continuous media data delivery on the three different kinds of network protocols such as CORBA, UDP/IP, and TCP/IP, with respect to QoS provisioning and throughput.     

视频服务器性能测试研究

视频点播是能向用户提供视频信息服务的应用系统,视频服务器是其中最关键的部分。随着技术的发展,用户对视频服务器的性能要求越来越高。但由于其实现方式上的差异,研究者并没有对视频服务器性能评价指标和测试方法形成共识。对此,文章提出并分析了视频服务器的各种评价指标,并据此给出了一个测试软件的实现方案。     

校园网数据平台规划设计中的若干问题研究

校园网数据平台包括服务器系统和存储系统,对服务器系统中的服务器选择标准、服务器管理以及机柜等服务器辅助设施的要求进行了阐述,对地方本科院校校园网存储需求和四种主流的存储技术进行了分析,结论是IP SAN是地方本科院校校园网存储系统的较好选择。     

一种分布式 ETL 工具的设计与实现

传统的ETL工具有集中执行、对服务器性能要求高等缺点,针对这些缺点,本文提出了一种基于Hadoop的分布式ETL系统。该系统在分布式文件系统基础上,利用相应的数据过滤器和Hadoop强大的并行处理能力,实现了集群分布式执行ETL流程。该分布式ETL系统具有较高的可扩展性和吞吐效率,同时能够自动实现负载均衡,执行效率高。     

校园网数据平台规划设计中的若干问题研究

校园网数据平台包括服务器系统和存储系统,对服务器系统中的服务器选择标准、服务器管理以及机柜等服务器辅助设施的要求进行了阐述,对地方本科院校校园网存储需求和四种主流的存储技术进行了分析,结论是IP SAN是地方本科院校校园网存储系统的较好选择.     

Load sharing in distributed multimedia-on-demand systems

Service providers have begun to offer multimedia-on-demand services to residential estates by installing isolated, small-scale multimedia servers at individual estates. Such an arrangement allows the service providers to operate without relying on a highspeed, large-capacity metropolitan area network, which is still not available in many countries. Unfortunately, installing isolated servers can incur very high server costs, as each server requires spare bandwidth to cope with fluctuations in user demand. The authors explore the feasibility of linking up several small multimedia servers to a (limited-capacity) network, and allowing servers with idle retrieval bandwidth to help out servers that are temporarily overloaded; the goal is to minimize the waiting time for service to begin. We identify four characteristics of load sharing in a distributed multimedia system that differentiate it from load balancing in a conventional distributed system. We then introduce a GWQ load sharing algorithm that fits and exploits these characteristics; it puts all servers' pending requests in a global queue, from which a server with idle capacity obtains additional jobs. The performance of the algorithm is captured by an analytical model, which we validate through simulations. Both the analytical and simulation models show that the algorithm vastly reduces wait times at the servers. The analytical model also provides guidelines for capacity planning. Finally, we propose an enhanced GWQ+L algorithm that allows a server to reclaim active local requests that are being serviced remotely. Simulation experiments indicate that the scheduling decisions of GWQ+L are optimal, i.e., it enables the distributed servers to approximate the performance of a large centralized server     

Analysis of task assignment policies in scalable distributedweb-server systems

A distributed multiserver Web site can provide the scalability necessary to keep up with growing client demand at popular sites. Load balancing of these distributed Web-server systems, consisting of multiple, homogeneous Web servers for document retrieval and a Domain Name Server (DNS) for address resolution, opens interesting new problems. In this paper, we investigate the effects of using a more active DNS which, as an atypical centralized scheduler, applies some scheduling strategy in routing the requests to the most suitable Web server. Unlike traditional parallel/distributed systems in which a centralized scheduler has full control of the system, the DNS controls only a very small fraction of the requests reaching the multiserver Web site. This peculiarity, especially in the presence of highly skewed load, makes it very difficult to achieve acceptable load balancing and avoid overloading some Web servers. This paper adapts traditional scheduling algorithms to the DNS, proposes new policies, and examines their impact under different scenarios. Extensive simulation results show the advantage of strategies that make scheduling decisions on the basis of the domain that originates the client requests and limited server state information (e.g., whether a server is overloaded or not). An initially unexpected result is that using detailed server information, especially based on history, does not seem useful in predicting the future load and can often lead to degraded performance