Adaptação ao Contexto em Sistemas de Comunicação Heterogéneos

Hoje em dia, as aplicações necessitam de ser concebidas para executar numa diversidade de dispositivos heterogéneos, desde servidores, PCs, computadores portáteis, PDAs, ou mesmo telemóveis. No caso de aplicações distribuídas, cada participante poderá estar em execução num dispositivo diferente. Perante esta diversidade, é cada vez mais importante desenhar e concretizar protocolos de comunicação adaptáveis, capazes de se reconfigurar, não só de acordo com o contexto local, mas também em função do contexto dos restantes participantes. Neste artigo é apresentada uma moldura de middleware que facilita o desenvolvimento e execução de protocolos adaptáveis em função do contexto. Esta moldura é ilustrada através de uma aplicação cooperativa, que se executa quer em dispositivos móveis quer fixos, e que beneficia da disponibilidade de uma pilha de comunicação em grupo reconfiguráve

On Statistically Estimated Optimistic Delivery inWide-Area Total Order Protocols

Total order broadcast protocols have been successfully applied as the basis for the construction of many fault-tolerant distributed systems. Unfortunately, the implementation of such a primitive can be expensive both in terms of communication steps and of number of messages exchanged. To alleviate this problem, optimistic total order protocols have been proposed. This paper addresses the problem of offering optimistic total order in geographically wide-area systems. We present a protocol that outperforms previous work, by minimizing the average latency of the optimistic notificatio

Run-time switching between total order algorithms

A total order protocol is a fundamental building block in the construction of many distributed fault-tolerant applications. Unfortunately, the implementation of such a primitive can be expensive both in terms of communication steps and of number of messages exchanged. This problem is exacerbated in large-scale systems, where the performance of the algorithm may be limited by the presence of high-latency links. Optimistic total order protocols have been proposed to alleviate this problem. However, different optimistic protocols offer quite distinct services. Moreover, there are certain algorithms that perform better in specific scenarios and given network properties. This dissertation provides an overview of different optimistic approaches and establishes a characterization of their properties and suitability to different execution environments. An adaptive protocol that is able to dynamically switch between different total order algorithms is proposed and evaluated. The protocol allows to achieve the best possible performance by supporting the reconfiguration such that, in each moment, the algorithm that is most appropriate to the present network conditions can be executed. Experimental results show that, using our protocol, adaptation can be achieved with negligible interference in the data flow.Tese de mestrado em Informática, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 200

Faculdade de Ciências da Universidade de Lisboa

are stored in PDF, with the report number as filename. Alternatively, reports are available by post from the above address

Context Adaptation of the Communication Stack

This paper presents a middleware framework to support the development of context-aware adaptive communication protocols, that can be reconfigured according not only to the local context, but also to the context of the remaining remote participants. The advantages of the framework are illustrated by using the concrete example of an adaptive group communication protocol. The protocol supports a distributed chat application that can be executed in both fixed PCs and mobile devices

Context Adaptation of the Communication Stack

This paper presents a middleware framework to support the development of context-aware adaptive communication protocols, that can be reconfigured according not only to the local context, but also to the context of the remaining remote participants. The advantages of the framework are illustrated by using the concrete example of an adaptive group communication protocol. The protocol supports a distributed chat application that can be executed in both fixed PCs and mobile device