Experiences in implementing a distributed object platform for multimedia applications

Two current trends in distributed computing are the emergence of standardized distributed object platforms such as CORBA, and the increasing use of continuous media data types. This paper describes and evaluates a platform which supports both standard CORBA interactions and continuous media interactions in a fully integrated environment. The platform is implemented as a self‐contained support infrastructure (GOPI core) and a separate ‘personality’ that sits on top of the infrastructure and provides a CORBA API. The platform user can create both request/reply oriented and stream oriented bindings with quality of service specifications that are honoured (as far as possible) by the infrastructure. A framework for quality of service management which involves the monitoring and adaptation of quality levels is also supported. The level of performance attained by the platform provides evidence of the feasibility of natively supporting continuous media in a distributed object platform environment. Copyright © 2000 John Wiley & Sons, Ltd.     

Mechanisms for object caching in distributed applications using Java RMI

Remote Method Invocation (RMI), a mechanism to access remote objects in Java‐based distributed applications, uses network communication for each method invocation. Consequently, using RMI in a wide‐area environment can cause poor application performance. One solution to improve performance is to cache the objects such that network communication is not necessary for each method invocation. In this paper, we present mechanisms to transparently add object caching to RMI. These mechanisms are compatible with existing RMI applications and use an event‐based model to support different consistency policies. The mechanisms also include the ability to adaptively select the consistency policy for an object based on its usage pattern. A novel feature of our mechanisms is the use of a ‘reduced object’, which is a partial representation of the RMI object. We experimentally evaluate and demonstrate the benefits of our mechanisms. Copyright © 2006 John Wiley & Sons, Ltd.     

Java based replicated server objects and their remote method invocations in a distributed environment

A distributed system is said to be fault‐tolerant if it is able to provide important services despite partial failures of the computers or software objects in the system. These systems are needed to support applications such as remote access and control, virtual mobile offices and wide area collaborative systems where there are chances of failures in the network and software objects. Fault‐tolerance is usually achieved by replicating the objects in the system. Traditional distributed applications constructed using Java RMI (remote method invocation) are not fault‐tolerant because of the lack of support of object replication. The objective of the present work is to design a remote method invocation that supports server object replication. To provide a fault‐tolerant service to the remote client objects, server objects are actively replicated. The problems associated with the method invocation in the context of active server object replication are presented and solutions are discussed and implemented. Copyright © 2001 John Wiley & Sons, Ltd.     

分布式对象模型应用技术研究

通过介绍分布式计算十大范型中的分布式对象模型,并针对该模型下的远程方法调用技术、对象请求代理及基于构件技术进行了较为深入的分析和比较,就RMI、CORBA、COM、DCOM及EJB进行了探讨。对分布式对象技术今后的发展作了展望。     

Introducing QoS awareness in distributed programming: QTcl

A number of distributed applications require communication services with quality of service (QoS) guarantees. Building global‐scale distributed systems with predictable properties is one of the great challenges for computer systems engineering in the new century. Work undertaken within the Internet Engineering Task Force has led to the definition of novel architectural models for the Internet with QoS support. According to these models, the network has to be appropriately configured in order to provide applications with the required performance guarantees. In next‐generation networks, enabling applications to interact with the underlying QoS services is of primary importance. Hence, several special‐purpose application programming interfaces (APIs) have been defined to let applications negotiate QoS parameters across QoS‐capable networks. However, so far, none of these APIs are available in different operating environments. We believe that such features should be embedded in programming environments for distributed applications. In this work we present how we included QoS control features in Tcl, a programming language that has been widely adopted for the development of distributed multimedia applications. Our work has led to the implementation of QTcl, an extended Tcl interpreter that provides programmers with a new set of primitives, in full compliance with the standard SCRAPI programming interface for the RSVP protocol. QTcl in highly portable, in that it enables standard QoS negotiation to be performed in a seamless fashion on the most common operating systems. Copyright © 2003 John Wiley & Sons, Ltd.     

Using execution trace data to improve distributed systems

One of the most challenging problems facing today's software engineer is to understand and modify distributed systems. One reason is that in actual use systems frequently behave differently than the developer intended. In order to cope with this challenge, we have developed a three‐step method to study the run‐time behavior of a distributed system. First, remote procedure calls are traced using CORBA interceptors. Next, the trace data is parsed to construct RPC call‐return sequences, and summary statistics are generated. Finally, a visualization tool is used to study the statistics and look for anomalous behavior. We have been using this method on a large distributed system (more than 500000 lines of code) with data collected during both system testing and operation at a customer's site. Despite the fact that the distributed system had been in operation for over three years, the method has uncovered system configuration and efficiency problems. Using these discoveries, the system support group has been able to improve product performance and their own product maintenance procedures. Copyright © 2002 John Wiley & Sons, Ltd.     

CORBA对象的复制技术研究

CORBA是一种当前流行的基于对象技术的中间件平台．CORBA对应用屏蔽了分布式系统的异构性．然而目前许多CORBA系统没有充分地考虑容错问题，而容错是运行在异构环境中的分布式应用的核心问题之一．因此，在最近几年对容错CORBA的研究成为CORBA技术的一个研究热点．本文讨论了对象复制技术的若干问题，提出了基于标准组通信机制的、能够容忍节点故障也能够容忍数值错误的基于选举的主动复制策略．     

容错CORBA系统的设计与实现

CORBA是基于对象技术的中间件平台的最流行的标准之一 .CORBA对应用屏蔽了分布式系统的异构性 .然而目前 CORBA还没有考虑容错问题 ,而容错是运行在异构环境中的分布式应用的核心问题之一 .因此 ,在最近几年许多为 CORBA应用添加可靠性和可用性的建议出现在各种文献上 .本文分析了这些建议的优、缺点 ,并提出了一个新颖的与 CORBA兼容的方法 ,这种方法不同于异步环境中可靠结构的分发方法 .     

分布式应用开发中的Java技术分析与比较

由于Java技术的面向对象、面向网络和数据库及跨平台等特性,使用Java技术来开发分布式应用系统就变得相当容易。现在应用比较广泛的3种主要的Java分布式计算技术,如RMI、EJB以及Jini具有各自不同的特点,对此进行了详细介绍,在此基础上,还对各自的异同点和使用范围进行了分析比较。     

Object‐oriented distributed computing based on remote class reference

Java RMI, Jini and CORBA provide effective mechanisms for implementing a distributed computing system. Recently many numeral libraries have been developed that take advantage of Java as an object‐oriented and portable language. The widely‐used client‐server method limits the extent to which the benefits of the object‐oriented approach can be exploited because of the difficulties arising when a remote object is the argument or return value of a remote or local method. In this paper this problem is solved by introducing a data object that stores the data structure of the remote object and related access methods. By using this data object, the client can easily instantiate a remote object, and use it as the argument or return value of either a local or remote method. Copyright © 2003 John Wiley & Sons, Ltd.     

一种基于负载均衡异构分布式系统的改进容错调度算法*

基于基/副版本技术提出了一种具有容错功能的静态进程调度算法。给出了一个新的设计模型,并在该模型上提出HDAL算法。此前类似负载均衡容错调度算法都是通过排序来解决故障发生前后的负载均衡调度问题。该算法与以往算法不同之处就是在不依赖排序情况下,通过引进控制进程来解决负载均衡调度问题,并且该算法的负载均衡性在一定程度上具有了可控性。最后通过模拟实验得到以下有意义的结论：在业务繁忙的异构系统中,HDAL算法比以往算法资源利用率高,负载均衡性更好,并且在调度速度上优势明显。     

Performance and effectiveness trade‐off for checkpointing in fault‐tolerant distributed systems

Checkpointing has a crucial impact on systems' performance and fault‐tolerance effectiveness: excessive checkpointing results in performance degradation, while deficient checkpointing incurs expensive recovery. In distributed systems with independent checkpoint activities there is no easy way to determine checkpoint frequencies optimizing response‐time and fault‐tolerance costs at the same time. The purpose of this paper is to investigate the potentialities of a statistical decision‐making procedure. We adopt a simulation‐based approach for obtaining performance metrics that are afterwards used for determining a trade‐off between checkpoint interval reductions and efficiency in performance. Statistical methodology including experimental design, regression analysis and optimization provides us with the framework for comparing configurations, which use possibly different fault‐tolerance mechanisms (replication‐based or message‐logging‐based). Systematic research also allows us to take into account additional design factors, such as load balancing. The method is described in terms of a standardized object replication model (OMG FT‐CORBA), but it could also be applied in other (e.g. process‐based) computational models. Copyright © 2006 John Wiley & Sons, Ltd.     

A Jini‐based software framework for developing distributed cooperative decision support systems

Many mission‐critical, decision‐making situations happen in dynamic, rapidly changing, and often unpredictable environments. Military, governmental, and medical contexts are examples of such situations, which can be characterized by highly decentralized, up‐to‐date data sets coming from various sources. Unlike other decision‐making tools, decision support systems (DSS) designed for such situations are challenged by the need to access this decentralized data at any time, from anywhere, under tight time constraints. This paper presents the design of a software framework for developing these kinds of distributed DSSs. The proposed Java‐based framework relies mostly on the Jini technology and its JavaSpaces service. The construction of the framework is presented with UML class diagrams, UML sequence diagrams, and additional explanations. Copyright © 2003 John Wiley & Sons, Ltd.     

支持容错QoS的高效分布式文件存储算法

针对现有存储算法无法兼顾文件定制化可靠性保证和系统良好可扩展性这一问题,提出一种全分布式的支持容错QoS(Quality of Service)的文件存储算法。算法允许文件被存储副本数量与其可靠性需求相匹配,无单一故障点与瓶颈点。此外,实验结果表明算法可保持节点间负载均衡,实现系统资源的高效利用。     

A QoS Based Reliable Routing Mechanism for Service Customization

Due to the rapid development of the Internet technology such as 5G/6G and artificial intelligence, more and more new network applications appear. Customers using these applications may have different individual demands and such a trend causes great challenges to the traditional integrated service and routing model. In order to satisfy the individual demands of customers, the service customization should be considered, during which the cost of Internet Service Provider (ISP) naturally increases. Hence, how to reach a balance between the customer satisfaction and the ISP profit becomes vitally important. Targeting at addressing this critical problem, this work proposes a service customization oriented reliable routing mechanism, which includes two modules, that is, the service customization module and the routing module. In particular, the former (i.e., the service customization module) is responsible for classifying services by analyzing and processing the customer’s demands. After that, the IPv6 protocol is used to implement the service customization, since it naturally supports differentiated services via the extended header fields. The latter is responsible for transforming the customized services into specific routing policies. Specifically, the Nash equilibrium based economic model is firstly introduced to make a perfect balance between the user satisfaction and the ISP profits, which could finally produce a win-win solution. After that, based on the customized service policies, an optimized grey wolf algorithm is designed to establish the routing path, during which the routing reliability is formulated and calculated. Finally, the experiments are carried out and the proposed mechanism is evaluated. The results indicate that the proposed service customization and routing mechanism improves the routing reliability, user satisfaction and ISP satisfaction by about 8.42%, 15.5% and 17.75% respectively compared with the classical open shortest path first algorithm and the function learning based algorithm.     

Self‐adaptation of service level in distributed systems

Modern distributed systems are diverse and dynamic, and consequently difficult to manage using traditional approaches, which rely on an extensive initial knowledge of the system. On the performance front, these systems often offer multiple opportunities for dynamically degrading or improving service level based on workload intensity, to avoid overload and underload. In this context, we propose a novel approach for building distributed systems capable of autonomously deciding when and how to adapt service level. Our approach limits the knowledge that must be provided manually to a component‐based representation of the system. From this representation, we build and maintain a performance profile, which allows us to (1) identify the most promising adaptations based on workload type and (2) dynamically characterize the intrinsic efficiency of each adaptation based on past attempts. We have successfully implemented and evaluated a prototype of our approach in the context of multi‐tiered application servers. Copyright © 2010 John Wiley & Sons, Ltd.     

MaDViWorld: a software framework for massively distributed virtual worlds

The MaDViWorld project represents an original attempt to define an appropriate software architecture for supporting massively distributed virtual world systems. A non‐massively distributed virtual world system is typically engineered as a client–server application for which a single server or more rarely a small cluster of servers contain all the world pertinent data and assume the world accessibility, consistency and persistence. On the client side, many of them enable interaction with the other users and the various objects of the world. The main originality of our approach resides in the fact that the server part of the proposed system is no more limited to a few centralized servers, but can be distributed on arbitrarily many of them. Indeed, MaDViWorld, the prototypal software framework already implemented using Java and RMI by our group, allows for creating the rooms of a given world on several machines, each running the server application. It is then possible to connect the rooms by way of simple doors and to populate them with active objects. Finally, avatars managed by the client application visit the rooms and interact with the active objects either directly on the remote host or locally by cloning or transporting them first to the client machine. This paper draws from the experience gained with the development of our first prototype in order to discuss, both at the user's and the vi‐world developer's level, the main software engineering issues related to the implementation of such massively distributed virtual world systems. Copyright © 2002 John Wiley & Sons, Ltd.     

Mutual exclusion in partitioned distributed systems

A network partition can break a distributed computing system into groups of isolated nodes. When this occurs, a mutual exclusion mechanism may be required to ensure that isolated groups do not concurrently perform conflicting operations. We study and formalize these mechanisms in three basic scenarios: where there is a single conflicting type of action; where there are two conflicting types, but operations of the same type do not conflict; and where there are two conflicting types, but operations of one type do not conflict among themselves. For each scenario, we present applications that require mutual exclusion (e.g., name servers, termination protocols, concurrency control). In each case, we also present mutual exclusion mechanisms that are more general and that may provide higher reliability than the voting mechanisms that have been proposed as solutions to this problem. Daniel Barbara is a graduate student in the Computer Science Department at Princeton University and expects to receive his Ph.D. Degree by July 1985. He obtained his BS in Electrical Engineering at the Universidad Metropolitana, Caracas, Venezuela in 1975. His research interests are Distributed Systems, Databases and Computer Networks. He is a member of IEEE and ACM. Hector Garcia-Molina is associate professor in the Department of Computer Science at Princeton University, Princeton, New Jersey. His research interests include distributed computing systems and database systems. He received a BS in electrical engineering from the Instituto Tecnologico de Monterrey. Mexico, in 1974. From Stanford University, Stanford, California, he received in 1975 a MS in electrical engineering and a PhD in computer science in 1979. Garcia-Molina is a member of the ACM and IEEE.     

Managing QoS in Multimedia Services

We present a case study of multimedia servicedelivery in the World Wide Web environment. Our systemis founded on the notion of end-to-endQuality-of-Service (QoS) management. We outline our viewof an integrated management frame and explain how ourQoS management architecture fits into this framework. Wethen describe our object-oriented multimedia deliverysystem based on CORBA and Java. We discuss some issues of stream delivery and managementwith reference to our own experiences.     

Composing and scheduling service‐oriented applications in time‐triggered distributed real‐time Java environments

During the last decade, the number of distributed application domains with temporal requirements has significantly augmented, arising the necessity of exploring new concepts and paradigms that allow, on the one hand, the development of dynamic and flexible distributed applications and, on the other hand, the reusability of code. Service‐oriented paradigms have been successfully applied to distributed environments, increasing their flexibility and allowing the reusability of their components. Besides, distributed real‐time Java technologies have shown to be a good candidate to deploy real‐time distributed applications. This paper presents a model for service‐oriented applications on a time‐triggered distributed real‐time Java environment, focusing on the definition of the temporal model of an application and its schedulability, applying and evaluating this model in real‐time service‐oriented composition algorithms. Copyright © 2012 John Wiley & Sons, Ltd.