WADE系统的多级分布式对象命名体系的设计与实现

WADE是一个基于校园网络的面向对象的元计算系统，通过分布式对象模型实现广域异构资源的一致管理，提出了一种4级对象层次结构支持模型的组织与实现，对象命名代理负责用户与系统的接口，映射用户对象标识符UOID到系统对象标识符SOID，对象通信代理为对象绑定通信地址，建立通信对象标识符COID，对象存储管理器则负责对象的惰性保存，并通过永久对象标识符POID识别惰性对象，WADE采用了双Cache缓冲机制，有利于提高对象的操作与通信性能。     

Reasoning about equations and functional dependencies on complexobjects

Virtually all semantic or object-oriented data models assume that objects have an identity separate from any of their parts, and allow users to define complex object types in which part values may be any other objects. This often results in a choice of query language in which a user can express navigating from one object to another by following a property value path. We consider a constraint language in which one may express equations and functional dependencies over complex object types. The language is novel in the sense that component attributes of individual constraints may correspond to property paths. The kind of equations we consider are also important, because they are a natural abstraction of the class of conjunctive queries for query languages that support property value navigation. In our introductory comments, we give an example of such a query and outline two applications of the constraint theory to problems relating to a choice of access plan for the query. We present a sound and complete axiomatization of the constraint language for the case in which interpretations are permitted to be infinite, where interpretations themselves correspond to a form of directed labeled graph. Although the implication problem for our form of equational constraint alone over arbitrary schema is undecidable, we present decision procedures for the implication problem for both kinds of constraints when the problem schema satisfies a stratification condition, and when all input functional dependencies are keys     

An efficient object promotion algorithm for persistent object systems

We report on a bulk object‐loading algorithm for persistent object stores called Ghosted Allocation. It allocates large numbers of objects in a persistent store atomically, efficiently, and reliably. Its main strengths are that it minimizes I/O traffic, optimizes the disk access pattern, and does not impose complex requirements on applications. Our experiments demonstrate that the Ghosted Allocation algorithm is efficient and most importantly scalable, sustaining allocation rates of up to 63 000 objects s^‐1. Copyright © 2001 John Wiley & Sons, Ltd.     

面向移动对象的高效组最近邻居查询方法

组最近邻居查询是移动对象数据库重要的查询类型之一。本文提出了一种基于网格索引结构的剪枝搜索策略,将空间区域划分为网格,通过对象点的网格单元标识减少组最近邻居查询所需要的节点访问代价。用步长迭代法得到查询对象集的质心,提出了一种移动对象组最近邻居查询MOGNN算法,采用更精确的裁剪搜索空间准则,减少了查询所需要访问的节点数目。实验结果与分析表明,基于网格索引的MOGNN查询算法具有良好的查询性能。     

Design a progressive video caching policy for video proxy servers

Proxy servers have been used to cache web objects to alleviate the load of the web servers and to reduce network congestion on the Internet. In this paper, a central video server is connected to a proxy server via wide area networks (WANs) and the proxy server can reach many clients via local area networks (LANs). We assume a video can be either entirely or partially cached in the proxy to reduce WAN bandwidth consumption. Since the storage space and the sustained disk I/O bandwidth are limited resources in the proxy, how to efficiently utilize these resources to maximize the WAN bandwidth reduction is an important issue. We design a progressive video caching policy in which each video can be cached at several levels corresponding to cached data sizes and required WAN bandwidths. For a video, the proxy server determines to cache a smaller amount of data at a lower level or to gradually accumulate more data to reach a higher level. The proposed progressive caching policy allows the proxy to adjust caching amount for each video based on its resource condition and the user access pattern. We investigate the scenarios in which the access pattern is priorly known or unknown and the effectiveness of the caching policy is evaluated.     

An object-oriented database system Jasmine: implementation,application, and extension

We have devised an object oriented DBMS called Jasmine for advanced applications. The paper describes the implementation, application and extension of Jasmine in detail. We focus on the impact of the design of its object oriented model and language on database implementation technology. We describe what part of traditional relational database technology we extend to handle object oriented features such as object identifiers, complex objects, class hierarchies, and methods. We introduce nested relations to efficiently store and access clustered complex objects. We use hash based methods to efficiently access nonclustered complex objects. We provide user defined functions directly evaluated on page buffers to efficiently process method invocation. We devise object oriented optimization of queries including class hierarchies, complex objects, and method invocation. We incorporate dedicated object buffering to allow efficient access to objects through object identifiers. We also describe nontrivial applications of Jasmine and discuss the validity of object oriented databases. We focus on a constraint management facility, which can be implemented by taking advantage of the extensibility of Jasmine. The facility includes constraint rules, called design goals, for automatic database population required by engineering applications. Finally, we describe a view facility for schema integration also needed by engineering applications in distributed environments. We focus on how we extend Jasmine to implement the facility     

Breadth-first ray tracing utilizing uniform spatial subdivision

Breadth-first ray tracing is based on the idea of exchanging the roles of rays and objects. For scenes with a large number of objects, it may be profitable to form a set of rays and compare each object in turn against this set. By doing so, thrashing, due to disk access, can be minimized. We present ways to combine breadth-first methods with traditional efficient algorithms, along with new schemes to minimize accessing objects stored on disk. Experimental analysis, including comparisons with depth-first ray tracing, shows that large databases can be handled efficiently with this approach     

Type-level access pattern view: Enhancing prefetching performance using the iterative and recursive patterns

Since recent applications such as XML applications, Geographical Information Systems (GIS), and CAD/CAM systems require highly efficient data management, they are built on Object-Relational DBMS (ORDBMS). The applications are called navigational applications, and they navigate the composite objects connected via the reference and the collection attributes in the ORDBMS. When a navigational application accesses an object, it first checks whether the object is stored in the object cache. If not, the object must be fetched from the database in the server, which is a costly operation. Prefetching identifies the objects that are most likely to be accessed in the near future by the navigational applications and stores them in the object cache in advance. Since prefetching reduces the number of object fetches, it is crucial for improving the application performance. However, the experimental result by Han et al. [16] showed that the improvement ratio of application performance is much lower than the reduction ratio of the number of object fetches.In this paper, we claim that the number of disk accesses in the server also considerably affects the application performance, and we propose a technique for minimizing disk accesses to improve the performance of the prefetch method by Han et al. [16] and hence the navigational application. The contributions of this paper are summarized as follows. (1) For the iterative and the recursive patterns, we propose methods for creating materialized views based on the type-level path access logs proposed in [15]. We refer to the materialized views as the type-level access pattern views. (2) We then present the algorithms for minimizing the number of disk accesses using the type-level access pattern views when prefetching the objects from the database in the server. (3) We present an implementation technique that, given a prefetch request from the client, quickly finds the most efficient type-level access pattern view. (4) We perform a series of experiments using a variety of databases to show that the proposed technique significantly improves the overall performance of the navigational application. Experimental result shows that we reduce the number of disk accesses by up to 33.0 times and improve the performance by up to 21.4 times.     

移动对象数据库预测范围聚集查询技术研究

针对预测范围聚集查询处理技术,提出了一种面向移动对象的聚集TPR树索引。聚集TPR树索引在TPR树中间节点中加入移动对象聚集信息以减少预测范围聚集查询所需要的节点访问代价。并增加了一个建于移动对象标识上的哈希辅助索引结构以支持自底向上的删除搜索算法,具有很好的动态更新性能和并发性。提出了一种EPRA查询算法,采用更精确的剪枝搜索准则,大大减少了查询所需要访问的磁盘节点,具有良好的查询性能。     

Memcached在呼叫中心系统中的应用

Memcached是一个高性能的分布式的内存对象缓存系统,它通过在内存中缓存数据对象来减少访问数据库的次数,从而提高了获取数据的速度。随着呼叫中心业务的不断扩展,业务数据量的增大,以及对呼叫中心实时性要求提高,Memcached成为符合系统设计要求的一项技术。首先简单介绍了Memcached及其特点,其次分析了呼叫中心的基本系统架构,最后给出了Memcached在呼叫中心中的设计与实现。     

Efficient segment-based video transcoding proxy for mobile multimedia services

To support various bandwidth requirements for mobile multimedia services for future heterogeneous mobile environments, such as portable notebooks, personal digital assistants (PDAs), and 3G cellular phones, a transcoding video proxy is usually necessary to provide mobile clients with adapted video streams by not only transcoding videos to meet different needs on demand, but also caching them for later use. Traditional proxy technology is not applicable to a video proxy because it is less cost-effective to cache the complete videos to fit all kinds of clients in the proxy. Since transcoded video objects have inheritance dependency between different bit-rate versions, we can use this property to amortize the retransmission overhead from transcoding other objects cached in the proxy. In this paper, we propose the object relation graph (ORG) to manage the static relationships between video versions and an efficient replacement algorithm to dynamically manage video segments cached in the proxy. Specifically, we formulate a transcoding time constrained profit function to evaluate the profit from caching each version of an object. The profit function considers not only the sum of the costs of caching individual versions of an object, but also the transcoding relationship among these versions. In addition, an effective data structure, cached object relation tree (CORT), is designed to facilitate the management of multiple versions of different objects cached in the transcoding proxy. Experimental results show that the proposed algorithm outperforms companion schemes in terms of the byte-hit ratios and the startup latency.     

A configurable RMI mechanism for sharing distributed Java objects

Many distributed programming environments have been designed to support distributed shared objects over the Internet. Most of these environments (Java RMI and CORBA, for example), support client-server applications where distributed objects reside on servers, which execute all methods (remote or local) invoked on the objects. Traditional client-server models do not support client-side object caching and the local access it provides. We believe that object caching is critical to distributed applications, especially over the Internet, where latency and bandwidth are highly variable. We have developed a configurable and efficient remote method invocation mechanism that provides the same interface as Java RMI, while extending its functionality so that shared objects can be cached on the accessing nodes. The mechanism, called Javanaise, is based on the caching of clusters, which are groups of interdependent Java objects. We have implemented a prototype consisting of a preprocessor that generates the required proxy classes from the application interfaces and a run-time environment that uses system classes to manage the consistency of cluster replicas cached on client nodes. We describe the motivation for the work, the design choices made for the Javanaise clustering mechanism, the implementation principles for managing Javanaise clusters, and the results from three experiments that compare the performance of Javanaise with Java RMI     

An adaptive navigation support system for conducting context-aware ubiquitous learning in museums

In context-aware ubiquitous learning, students are guided to learn in the real world with personalized supports from the learning system. As the learning resources are realistic objects in the real world, certain physical constraints, such as the limitation of stream of people who visit the same learning object, the time for moving from one object to another, and the environmental parameters, need to be taken into account. Moreover, the values of these context-dependent parameters are likely to change swiftly during the learning process, which makes it a challenging and important issue to find a navigation support mechanism for suggesting learning paths for individual students in real time. In this paper, the navigation support problem for context-aware ubiquitous learning is formulated and two navigation support algorithms are proposed by taking learning efficacy and navigation efficiency into consideration. From the simulation results of learning in a butterfly museum setting, it is concluded that the innovative approach is helpful to the students to more effectively and efficiently utilize the learning resources and achieve better learning efficacy.     

Resilient objects in broadband networks

An object is said to be resilient if operations on the object can be performed even if some nodes of the network fail. To support resiliency, copies of the objects are stored on different nodes, and access to different copies is coordinated. The properties of broadcast networks are utilized to devise a distributed scheme for implementing resilient objects. All the copies of an object are equivalent. If an operation is requested on an object, the operation is performed on all the copies of the object. No special mechanisms are needed if some copies are not available due to node failures, as long as there is at least one active node that has a copy of the object and the network does not get partitioned. Simulation results indicate that the number of messages needed to perform an operation increases slowly and the response time for performing an operation decreases as the number of copies increases     

基于对象访问层优化JDO的持久对象查询

JDO是Java环境中一种面向对象持久存储技术,通过在其持久实现的业务层与持久层之间增加对象访问层,对于业务查询可通过该层中索引对象解析成对对象D的查询,以减少对象处理时间和磁盘IO次数,并通过模拟实验分析了不同过滤因子对查询效率的影响,证明了本方案在索引因子较小的情况下能节省磁盘访问时间.     

基于层次结构的元数据动态管理方法的研究

在大规模分布式存储系统中,元数据高性能服务和扩展性已成为一个重要的研究热点.在元数据服务器(metadata server,MDS)中,将元数据分解为目录对象和文件对象.目录对象为定位性元数据,提供文件所在位置和访问控制;文件对象为描述性元数据,描述文件的数据特性.每个MDS负责所有目录对象和自身的文件对象,同时,以目录对象ID和文件名为关键字的Hash值作为局部元数据查找表的索引,通过Bloom Filter算法将每个MDS的局部元数据查找表压缩成一个摘要,这样既可利用MDS中Cache,提高Cache的命中率,减少磁盘I/O次数,动态扩展MDS,又能够实现快速的元数据查找.     

An Effective Cache Replacement Algorithm in Transcoding-Enabled Proxies

In this paper, we address the problem of cache replacement for transcoding proxy caching. Transcoding proxy is a proxy that has the functionality of transcoding a multimedia object into an appropriate format or resolution for each client. We first propose an effective cache replacement algorithm for transcoding proxy. In general, when a new object is to be cached, cache replacement algorithms evict some of the cached objects with the least profit to accommodate the new object. Our algorithm takes into account of the inter-relationships among different versions of the same multimedia object, and selects the versions to replace according to their aggregate profit which usually differs from simple summation of their individual profits as assumed in the existing algorithms. It also considers cache consistency, which is not considered in the existing algorithms. We then present a complexity analysis to show the efficiency of our algorithm. Finally, we give extensive simulation results to compare the performance of our algorithm with some existing algorithms. The results show that our algorithm outperforms others in terms of various performance metrics.     

PPP: Prefix-Based Popularity Prediction for Efficient Content Caching in Contentcentric Networks

In the Content-Centric Networking (CCN) architecture, popular content can be cached in some intermediate network devices while being delivered, and the following requests for the cached content can be efficiently handled by the caches. Thus, how to design in-network caching is important for reducing both the traffic load and the delivery delay. In this paper, we propose a caching framework of Prefix-based Popularity Prediction (PPP) for efficient caching in CCN. PPP assigns a lifetime (in a cache) to the prefix of a name (of each cached object) based on its access history (or popularity), which is represented as a Prefix-Tree (PT). We demonstrate PPP’s predictability of content popularity in CCN by both traces and simulations. The evaluation results show that PPP can achieve higher cache hits and less traffic load than traditional caching algorithms (i.e., LRU and LFU). Also, its performance gain increases with users of high mobility.     

一种有效的混合式P2P Web缓存系统HCache

针对当前P2P Web 缓存系统中副本过多的问题,提出了一种有效的混合式P2P Web缓存系统：HCache。HCache根据用户对网页的访问特点及网页的优先级,对网页进行有选择的缓存,进而减少P2P Web缓存系统中的副本个数。根据Web对象当前的流行度,对LRU替换策略进行了改进(ELRU),提高了P2P Web缓存的命中率。在日志驱动的模拟实验表明,HCache缓存系统提高了Web缓存的命中率和性能。     

Issues in the design of a storage server for video-on-demand

We examine issues related to the design of a storage server for video-on-demand (VOD) applications. The storage medium considered is magnetic disks or arrays of disks. We investigate disk scheduling policies, buffer management policies and I/O bus protocol issues. We derive the number of sessions that can be supported from a single disk or an array of disks and determine the amount of buffering required to support a given number of users. Furthermore, we propose a scheduling mechanism for disk accesses that significantly lowers the buffer-size requirements in the case of disk arrays. The buffer size required under the proposed scheme is independent of the number of disks in the array. This property allows for striping video content over a large number of disks to achieve higher concurrency in access to a particular video object. This enables the server to satisfy hundreds of independent requests to the same video object or to hundreds of different objects while storing only one copy of each video object. The reliability implications of striping content over a large number of disks are addressed and two solutions are proposed. Finally, we examine various policies for dealing with disk thermal calibration and the placement of videos on disks and disk arrays.