Symmetric Replication for Structured Peer-to-Peer Systems

Structured peer-to-peer systems rely on replication as a basic means to provide fault-tolerance in presence of high churn. Most select replicas using either multiple hash functions, successor-lists, or leaf-sets. We show that all three alternatives have limitations. We present and provide full algorithmic speci¯cation for a generic replication scheme called symmetric replication which only needs O(1) message for every join and leave operation to maintain any replication degree. The scheme is applicable to all existing structured peer-to-peer systems, and can be implemented on-top of any DHT. The scheme has been implemented in our DKS system, and is used to do load-balancing, end-to-end fault-tolerance, and to increase the security by using distributed voting. We outline an extension to the scheme, implemented in DKS, which adds routing proximity to reduce latencies. The scheme is particularly suitable for use with erasure codes, as it can be used to fetch a random subset of the replicas for decoding

Multicast in DKS(N, k, f) Overlay Networks

Recent developments in the area of peer-to-peer computing show that structured overlay networks implementing distributed hash tables scale well and can serve as infrastructures for Internet scale applications. We are developing a family of infrastructures, DKS(N; k; f), for the construction of peer-to-peer applications. An instance of DKS(N; k; f) is an overlay network that implements a distributed hash table and which has a number of desirable properties: low cost of communication, scalability, logarithmic lookup length, fault-tolerance and strong guarantees of locating any data item that was inserted in the system. In this paper, we show how multicast is achieved in DKS(N, k, f) overlay networks. The design presented here is attractive in three main respects. First, members of a multicast group self-organize in an instance of DKS(N, k, f) in a way that allows co-existence of groups of different sizes, degree of fault-tolerance, and maintenance cost, thereby, providing flexibility. Second, each member of a group can multicast, rather than having single source multicast. Third, within a group, dissemination of a multicast message is optimal under normal system operation in the sense that there are no redundant messages despite the presence of outdated routing information

Self-Correcting Broadcast in Distributed Hash Tables

We present two broadcast algorithms that can be used on top of distributed hash tables (DHTs) to perform group communication and arbitrary queries. Unlike other P2P group communication mechanisms, which either embed extra information in the DHTs or use random overlay networks, our algorithms take advantage of the structured DHT overlay networks without maintaining additional information. The proposed algorithms do not send any redundant messages. Furthermore the two algorithms ensure 100% coverage of the nodes in the system even when routing information is outdated as a result of dynamism in the network. The first algorithm performs some correction of outdated routing table entries with a low cost of correction traffic. The second algorithm exploits the nature of the broadcasts to extensively update erroneous routing information at the cost of higher correction traffic. The algorithms are validated and evaluated in our stochastic distributed-algorithms simulator

The essence of P2P: A reference architecture for overlay networks

The success of the P2P idea has created a huge diversity of approaches, among which overlay networks, for example, Gnutella, Kazaa, Chord, Pastry, Tapestry, P-Grid, or DKS, have received specific attention from both developers and researchers. A wide variety of algorithms, data structures, and architectures have been proposed. The terminologies and abstractions used, however, have become quite inconsistent since the P2P paradigm has attracted people from many different communities, e.g., networking, databases, distributed systems, graph theory, complexity theory, biology, etc. In this paper we propose a reference model for overlay networks which is capable of modeling different approaches in this domain in a generic manner. It is intended to allow researchers and users to assess the properties of concrete systems, to establish a common vocabulary for scientific discussion, to facilitate the qualitative comparison of the systems, and to serve as the basis for defining a standardized API to make overlay networks interoperable

Hypotyposis and Objectification of the Horrible Scene: Stylizing the Figure of Reality in Some Contemporary Francophone African Novels

This study shows that it is simplistic to argue that the narrative dynamics in contemporary Francophone African novel is monotonous. It then consists in demonstrating that it is rather indicative of a striking realism that it sometimes offers to see horrific scenes of life exhibited such that the reader immediately succumb to the temptation to share the worldview of author in review through the characters involved. Based on the operationalization of the explicit-implicit duality as structured in Pierre Barberis’s sociocriticism framework, the study postulates, in two parts, that hypotyposis is a fertile and multiple variable figure which is spread at the same time like the modalisation of an event, a relevant semiological sign, a figure of the transcendence convened by the novelist in order to collect the attention of the reader by touching his psyche and causing in him a certain number of emotions such as the feeling of horror.La présente étude montre qu’il est réducteur de soutenir que la dynamique de narration dans le roman africain francophone contemporain est monocorde. Il s’agit alors de la monstration qu’elle est plutôt révélatrice d’un réalisme si saisissant qu’il offre parfois à voir des scènes de vie horribles exhibées telles que le lecteur succombe immédiatement à la tentation de partager la vision du monde de l’auteur en examen par le biais des personnages mis en jeu. Fondée sur l’opérationnalisation du diptyque explicite-implicite structuré à travers la sociocritique de Pierre Barbéris, l’étude postule, en deux parties, que l’hypotypose est une figure féconde à variable multiple qui se déploie à la fois comme la modalisation d’un événement, un signe sémiologique pertinent, une figure de la transcendance convoquée par le romancier en vue de capter l’attention du lecteur et de toucher sa psyché en suscitant en lui un certain nombre d’émotions, par exemple le sentiment de l’horrible

Intensification et gestion des espaces ruraux. Compte rendu des discussions de la séance plénière II

Cette séance était centrée sur l'analyse des pratiques des agriculteurs et des éleveurs au Nord-Cameroun et leurs conséquences sur le revenu des producteurs, les relations entre les différents acteurs et le maintien de la fertilité du milieu. Les exposés ont mis en lumière les axes prioritaires d'intervention en matière d'élevage. Les exposés sur les systèmes de culture posent deux problèmes : l'appropriation foncière et le maintien des capacités de production de la zone d'installation des migrants. L'étude du fonctionnement des systèmes de culture en zone d'installation de migrants a montré les stratégies extensives des paysans. Le besoin d'un cadre de planification régionale et de concertation entre les acteurs est souligné. La recherche doit étudier les aspects fonciers au Nord-Cameroun dans leurs dimensions sociales, historiques et politiques

Le contrat d’adhésion : actualités et droit international privé

Le contrat d’adhésion occupe une place de plus en plus importante dans les relations contractuelles contemporaines. Outil d’efficacité et de rapidité, il est en outre favorisé par la technique numérique. En effet, les contrats conclus par l’entremise de pages ou de sites Web ne peuvent se former autrement. Que ce soit sur le plan national ou dans le contexte de relations internationales, une très grande partie des contrats de vente actuels sont des contrats d’adhésion.Or ce type de contrat recèle indiscutablement des dangers pour l’adhérent, quelle que soit sa qualité (professionnel ou simple particulier). À cet égard, la notion de clause abusive vient immédiatement à l’esprit.Après avoir passé en revue les figures actuelles du contrat d’adhésion, les auteurs se penchent sur son régime dans les relations internationales. D’une part, ils examinent les protections de l’adhérent dans une relation internationale et prônent une protection de tout adhérent, pas seulement de l’adhérent consommateur, comme a choisi de le faire le codificateur québécois. D’autre part, ils étudient les clauses de droit international privé — clauses de rattachement juridictionnel et clauses de rattachement normatif — et proposent des solutions pour protéger l’adhérent dans les cas où de telles clauses lui sont défavorables.The contract of adhesion continues to acquire ever greater importance in today’s contractual relations. Its strength lies in its efficiency, speed and seamless adaptation to digital processing techniques. Indeed, deals contracted over Web sites or pages could not be concluded in any other way. Whether in a domestic or international setting, the lion’s share of current sales agreements are contracts of adhesion.Obviously, this type of contract can harbour many dangers for unwary contracting parties, be they professionals or consumers. In these circumstances, the concept of the abusive clause immediately comes to mind.In their research into this issue, the authors begin by a review of the current types of contracts of adhesion and then examine the rules applicable to them in an international setting. First, they look into what protections the adhering party has in an international transaction and then suggest protection for all adhering parties, and not merely consumers, as the Québec codifiers have chosen to do. Furthermore, they analyse clauses in relation to private international law — jurisdictional and normative — in order to set forth solutions to protect the adhering party in cases where such clauses are unfavourable to the latter