Concretização de protocolos com fiabilidade semântica

Os protocolos de difusão são muito utilizados na construção de sistemas distribuídos. Neste contexto,a habilidade semântica é um novo modelo de coerência introduzido recentemente que explora o conceito de obsolescência:uma mensagem torna-se obsoleta quando o seu conteúdo. É sobreposto por uma outra mensagem mais recente. O conhecimento de quais as mensagens obsoletas pode ser usado para libertar recursos, aumentando o desempenho do sistema. Este artigo aborda os desafios de realizar uma concretização modular de uma pilha de protocolos oferecendo habilidade semântica. Esta concretização foi realizada usando uma moldura de objectos de suporte composição e execução de micro-protocolos. Ao contrário do que acontece em concretizáveıs monolíticas, numa concretização modular as mensagens em processamento na pilha de protocolos encontram-se dispersas por diferentes camadas, dificultando a tarefa de aplicar relações de obsolescência

Reducing the cost of group communication with semantic view synchrony

View Synchrony (VS) is a powerful abstraction in the design and implementation of de- pendable distributed systems. By ensuring that processes deliver the same set of messages in each view, it allows them to maintain consistency across membership changes. However, experience indicates that it is hard to combine strong reliability guarantees as offered by VS with stable high performance. In this paper we propose a novel abstraction, Semantic View Synchrony (SVS), that exploits the application's semantics to cope with high throughput applications. This is achieved by allowing some messages to be dropped while still preserving consistency when new views are installed. Thus, SVS inherits the elegance of view synchronous communi- cation. The paper describes how SVS can be implemented and illustrates its usefulness in the context of distributed multi-player games

Hybrid Dissemination: Adding Determinism to Probabilistic Multicasting in Large-Scale P2P Systems

Abstract. Epidemic protocols have demonstrated remarkable scalability and robustness in disseminating information on internet-scale, dynamic P2P systems. However, popular instances of such protocols suffer from a number of significant drawbacks, such as increased message overhead in push-based systems, or low dissemination speed in pull-based ones. In this paper we study push-based epidemic dissemination algorithms, in terms of hit ratio, communication overhead, dissemination speed, and resilience to failures and node churn. We devise a hybrid push-based dissemination algorithm, combining probabilistic with deterministic properties, which limits message overhead to an order of magnitude lower than that of the purely probabilistic dissemination model, while retaining strong probabilistic guarantees for complete dissemination of messages. Our extensive experimentation shows that our proposed algorithm outperforms that model both in static and dynamic network scenarios, as well as in the face of large-scale catastrophic failures. Moreover, the proposed algorithm distributes the dissemination load uniformly on all participating nodes. Keywords: Epidemic/Gossip protocols, Information Dissemination, Peer-to-Peer

Enforcing Strong Consistency with Semantically View Synchronous Multicast

Replication is a fundamental strategy to obtain highly available services. Among the mechanisms that support replication, view synchronous multicast protocols emerge as a powerful abstraction to encapsulate fundamental problems in replication. Unfortunately, in the presence of a temporarily slow processor or network link the performance of implementations of view synchronous multicast is degraded for the whole group. This happens due to the strong reliability criterion, which forces a potentially large number of messages to be stored, eventually leading to buffer exhaustion and intermittent blocking of the application. This paper proposes a new multicast primitive, Semantically View Synchronous Multicast, that alleviates this problem by selectively weakening reliability constraints while, at the same time, allowing strong consistency to be enforced at a higher level. The usefulness and practical relevance of the new primitive is illustrated using a modified primary-backup replication protoco

Semantically reliable multicast: definition, implementation and performance evaluation

Semantic reliability is a novel correctness criterion for multicast protocols based on the concept of message obsolescence: A message becomes obsolete when its content or purpose is superseded by a subsequent message. By exploiting obsolescence, a reliable multicast protocol may drop irrelevant messages to find additional buffer space for new messages. This makes the multicast protocol more resilient to transient performance perturbations of group members, thus improving throughput stability. This paper describes our experience in developing a suite of semantically reliable protocols. It summarizes the motivation, definition, and algorithmic issues and presents performance figures obtained with a running implementation. The data obtained experimentally is compared with analytic and simulation models. This comparison allows us to confirm the validity of these models and the usefulness of the approach. Finally, the paper reports the application of our prototype to distributed multiplayer games.POSI/32869/CHS/200

Lightweight Probabilistic Broadcast

The growing interest in peer-to-peer applications has underlined the importance of scalability in modern distributed systems. Not surprisingly, much research effort has been invested in gossip-based broadcast protocols. These trade the traditional strong reliability guarantees against very good ``scalability'' properties. Scalability is in that context usually expressed in terms of throughput, but there is only little work on how to reduce the overhead of membership management at large scale. This paper presents Lightweight Probabilistic Broadcast (lpbcast), a novel gossip-based broadcast algorithm which preserves the inherent throughput scalability of traditional gossip-based algorithms and adds a notion of membership management scalability: every process only knows a random subset of fixed size of the processes in the system. We formally analyze our broadcast algorithm in terms of scalability with respect to the size of individual views, and compare the analytical results both with simulations and concrete measurements

Hierarchical Probabilistic Multicast

A VENIR

Scalability, Throughput Stability and Efficient Buffering in ReliableMulticast Protocols

This study investigates the issues of scalability, throughput stability and efficient buffering in reliable multicast protocols. The focus is on a new class of scalable reliable multicast protoco, PBcast that is based on an epidemic loss recovery mechanism. The protocol offers scalability, throughput stability and a bimodal delivery guarantee as the key features. A theoretical analysis study for the protocol is already available