Efficient simulation of view synchrony

This report presents an algorithm for efficiently simulating view synchrony, including failure-atomic total-order multicast in a discrete-time event simulator. In this report we show how a view synchrony implementation tailored to a simulated environment removes the need for third party middleware and detailed network simulation, thus reducing the complexity of a test environment. An additional advantage is that simulated view synchrony can generate all timing behaviours allowed by the model instead of just those exhibited by a particular view synchrony implementation

Multimedia Multicast Transport Service for

Reliability carries different meanings for different applications. For example, in a replicated database setting, reliability means that messages are never lost, and that messages arrive in the same order at all sites. In order to guarantee this reliability property, it is acceptable to sacrifice real-time message delivery: some messages may be greatly delayed, and at certain periods message transmission may even be blocked. While this is perfectly acceptable behavior for a reliable database application, this behavior is intolerable for a reliable video server. For a continuous MPEG video player [20, 19], reliability means real-time message delivery, at a certain bandwidth; It is acceptable for some messages to be lost, as long as the available bandwidth complies with certain predetermined stochastic assumptions. Introducing database style reliability (i.e. message recovery and order constraints) may violate these assumptions, rendering the MPEG decoding algorithm incorrect. Many CSCW groupware and multimedia applications require quality of service multicast for most of their messages, and may greatly benefit from reliable multicast for a small portion of “critical ” messages. Furthermore, such applications often need to be fault-tolerant, and need to support smooth reconfiguration when parties join or leave

Spatiotemporal Multicast and Partitionable Group Membership Service

The recent advent of wireless mobile ad hoc networks and sensor networks creates many opportunities and challenges. This thesis explores some of them. In light of new application requirements in such environments, it proposes a new multicast paradigm called spatiotemporal multicast for supporting ad hoc network applications which require both spatial and temporal coordination. With a focus on a special case of spatiotemporal multicast, called mobicast, this work proposes several novel protocols and analyzes their performances. This dissertation also investigates implications of mobility on the classical group membership problem in distributed computing, proposes a new specification for a partitionable group membership service catering to applications on wireless mobile ad hoc networks, and provides a mobility-aware algorithm and middleware for this service. The results of this work bring new insights into the design and analysis of spatiotemporal communication protocols and fault-tolerant computing in wireless mobile ad hoc networks

Group communication as a base for a load-balancing replicated data service

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (p. 109-111).by Roger I. Khazan.M.S

A software architecture for consensus based replication

Orientador: Luiz Eduardo BuzatoTese (doutorado) - Universidade Estadual de Campinas, Instituto de ComputaçãoResumo: Esta tese explora uma das ferramentas fundamentais para construção de sistemas distribuídos: a replicação de componentes de software. Especificamente, procuramos resolver o problema de como simplificar a construção de aplicações replicadas que combinem alto grau de disponibilidade e desempenho. Como ferramenta principal para alcançar o objetivo deste trabalho de pesquisa desenvolvemos Treplica, uma biblioteca de replicação voltada para construção de aplicações distribuídas, porém com semântica de aplicações centralizadas. Treplica apresenta ao programador uma interface simples baseada em uma especificação orientada a objetos de replicação ativa. A conclusão que defendemos nesta tese é que é possível desenvolver um suporte modular e de uso simples para replicação que exibe alto desempenho, baixa latência e que permite recuperação eficiente em caso de falhas. Acreditamos que a arquitetura de software proposta tem aplicabilidade em qualquer sistema distribuído, mas é de especial interesse para sistemas que não são distribuídos pela ausência de uma forma simples, eficiente e confiável de replicá-losAbstract: This thesis explores one of the fundamental tools for the construction of distributed systems: the replication of software components. Specifically, we attempted to solve the problem of simplifying the construction of high-performance and high-availability replicated applications. We have developed Treplica, a replication library, as the main tool to reach this research objective. Treplica allows the construction of distributed applications that behave as centralized applications, presenting the programmer a simple interface based on an object-oriented specification for active replication. The conclusion we reach in this thesis is that it is possible to create a modular and simple to use support for replication, providing high performance, low latency and fast recovery in the presence of failures. We believe our proposed software architecture is applicable to any distributed system, but it is particularly interesting to systems that remain centralized due to the lack of a simple, efficient and reliable replication mechanismDoutoradoSistemas de ComputaçãoDoutor em Ciência da Computaçã

Towards implementing group membership in dynamic networks : a performance evaluation study

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (p. 105-109).Support for dynamic groups is an integral part of the U.S. Department of Defense's vision of Network-Centric Operations. Group membership (GM) serves as the foundation of many group-oriented systems; its fundamental role in applications such as reliable group multicast, group key management, data replication, and distributed collaboration, makes optimization of its efficiency important. The impact of GM's performance is amplified in dynamic, failure-prone environments with intermittent connectivity and limited bandwidth, such as those that host military on the move operations. A recent theoretical result has proposed a novel GM algorithm, called Sigma, which solves the Group Membership problem within a single round of message exchange. In contrast, all other GM algorithms require more rounds in the worst case. Sigma's breakthrough design both makes and handles tradeoffs between fast agreement and possible transient disagreement, raising the question: how efficiently and accurately does Sigma perform in practice? We answer this question by implementing and studying Sigma in simulation, as well as two leading GM algorithms - Moshe and Ensemble - in a comparative performance analysis. Among the variants of Sigma that we study is Leader-Based Sigma, which we design as a more scalable alternative.(cont.) We also discuss parameters enabling Sigma's optimal practical deployment in a variety of applications and environments. Our simulations show that, consistently with theoretical results, Sigma always terminates within a single round of message exchange, faster than Moshe and Ensemble. Moreover, Sigma has less message overhead and produces virtually the same quality of views as Moshe and Ensemble, when used with a filter for limiting disagreement. These results strongly indicate that Sigma is not just a theoretical result, but indeed a result with important practical implications for Group Communication Systems: the efficiency of GM applications can be significantly improved, without compromising accuracy, by replacing current GM algorithms with Sigma.by Sophia Yuditskaya.M.Eng

Using simulation techniques to prove timing properties

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 151-160).by Victor Luchangco.M.S

Reliable and Real-Time Distributed Abstractions

The celebrated distributed computing approach for building systems and services using multiple machines continues to expand to new domains. Computation devices nowadays have additional sensing and communication capabilities, while becoming, at the same time, cheaper, faster and more pervasive. Consequently, areas like industrial control, smart grids and sensor networks are increasingly using such devices to control and coordinate system operations. However, compared to classic distributed systems, such real-world physical systems have different needs, e.g., real-time and energy efficiency requirements. Moreover, constraints that govern communication are also different. Networks become susceptible to inevitable random losses, especially when utilizing wireless and power line communication. This thesis investigates how to build various fundamental distributed computing abstractions (services) given the limitations, the performance and the application requirements and constraints of real-world control, smart grid and sensor systems. In quest of completeness, we discuss four distributed abstractions starting from the level of network links all the way up to the application level. At the link level, we show how to build an energy-efficient reliable communication service. This is especially important for devices with battery-powered wireless adapters where recharging might be unfeasible. We establish transmission policies that can be used by processes to decide when to transmit over the network in order to avoid losses and minimize re-transmissions. These policies allow messages to be reliably transmitted with minimum transmission energy. One level higher than links is failure detection, a software abstraction that relies on communication for identifying process crashes. We prove impossibility results concerning implementing classic eventual failure detectors in networks with probabilistic losses. We define a new implementable type of failure detectors, which preserves modularity. This means that existing deterministic algorithms using eventual failure detectors can still be used to solve certain distributed problems in lossy networks: we simply replace the existing failure detector with the one we define. Using failure detectors, processes might get information about failures at different times. However, to ensure dependability, environments such as distributed control systems (DCSs), require a membership service where processes agree about failures in real time. We prove that the necessary properties of this membership cannot be implemented deterministically, given probabilistic losses. We propose an algorithm that satisfies these properties, with high probability. We show analytically, as well as experimentally (within an industrial DCS), that our technique significantly enhances the DCS dependability, compared to classic membership services, at low additional cost. Finally, we investigate a real-time shared memory abstraction, which vastly simplifies programming control applications. We study the feasibility of implementing such an abstraction within DCSs, showing the impossibility of this task using traditional algorithms that are built on top of existing software blocks like failure detectors. We propose an approach that circumvents this impossibility by attaching information to the failure detection messages, analyze the performance of our technique and showcase ways of adapting it to various application needs and workloads

Strong and Weak Virtual Synchrony in Horus

A formal definition of {\em strong virtual synchrony}, capturing the semantics of virtual synchrony as implemented in Horus, is presented. This definition has the nice property that every message is delivered within the view in which it was sent. However, it is shown that in order to implement strong virtual synchrony, the application program has to block messages during view changes. An alternative definition, called {\em weak virtual synchrony}, which can be implemented without blocking messages, is then presented. This definition still guarantees that messages will be delivered within the view in which they were sent, only that it uses a slightly weaker notion of what the view in which a message was sent is. An implementation of weak virtual synchrony that does not block messages during view changes is developed, and it is shown how to use a system that provides weak virtual synchrony even when strong virtual synchrony is actually needed. To capture additional ordering requirements, the definition of {\em ordered virtual synchrony} is presented. Finally, it is discussed how to extend the definitions in order to cope with the fact that a process can become a member of more than one group