Adaptive Replication in Distributed Content Delivery Networks

We address the problem of content replication in large distributed content delivery networks, composed of a data center assisted by many small servers with limited capabilities and located at the edge of the network. The objective is to optimize the placement of contents on the servers to offload as much as possible the data center. We model the system constituted by the small servers as a loss network, each loss corresponding to a request to the data center. Based on large system / storage behavior, we obtain an asymptotic formula for the optimal replication of contents and propose adaptive schemes related to those encountered in cache networks but reacting here to loss events, and faster algorithms generating virtual events at higher rate while keeping the same target replication. We show through simulations that our adaptive schemes outperform significantly standard replication strategies both in terms of loss rates and adaptation speed.Comment: 10 pages, 5 figure

Exploring heterogeneity of unreliable machines for p2p backup

P2P architecture is a viable option for enterprise backup. In contrast to dedicated backup servers, nowadays a standard solution, making backups directly on organization's workstations should be cheaper (as existing hardware is used), more efficient (as there is no single bottleneck server) and more reliable (as the machines are geographically dispersed). We present the architecture of a p2p backup system that uses pairwise replication contracts between a data owner and a replicator. In contrast to standard p2p storage systems using directly a DHT, the contracts allow our system to optimize replicas' placement depending on a specific optimization strategy, and so to take advantage of the heterogeneity of the machines and the network. Such optimization is particularly appealing in the context of backup: replicas can be geographically dispersed, the load sent over the network can be minimized, or the optimization goal can be to minimize the backup/restore time. However, managing the contracts, keeping them consistent and adjusting them in response to dynamically changing environment is challenging. We built a scientific prototype and ran the experiments on 150 workstations in the university's computer laboratories and, separately, on 50 PlanetLab nodes. We found out that the main factor affecting the quality of the system is the availability of the machines. Yet, our main conclusion is that it is possible to build an efficient and reliable backup system on highly unreliable machines (our computers had just 13% average availability)

Access Control in Weakly Consistent Systems

Eventually consistent models have become popular in the last years in data storage systems for cloud environments, allowing to give users better availability and lower latency. In this model, it is possible for replicas to be temporarily inconsistent, having been proposed various solutions to deal with this inconsistency and ensure the final convergence of data. However, defining and enforcing access control policies under this model is still an open challenge. The implementation of access control policies for these systems raises it’s own challenges, given the information about the permissions is itself kept in a weakly consistent form. In this dissertation, a solution for this problem is proposed, that allows to prevent the non authorized access and modification of data. The proposed solution allows concurrent modifications on the security policies, ensuring their convergence when they are used to verify and enforce access control the associated data. In this dissertation we present an evaluation of the proposed model, showing the solution respects the correct functioning over possible challenging situations, also discussing its application on scenarios that feature peer-to-peer communication between clients and additional replicas on the clients, with the goal of providing a lower latency and reduce the load on centralized components

Economic interests and the origins of electoral systems

Die gängige Begründung – basierend auf der bahnbrechenden Arbeit Rokkans – dafür, dass ein spezifisches Wahlsystem bevorzugt wird, ist, dass die Verhältniswahl („proportional respresentation“ oder „PR“) von einer zersplitterten Rechte eingeführt wurde, um ihre Klasseninteressen gegenüber denen einer wachsenden Linken zu verteidigen. Neue Erkenntnisse zeigen jedoch, dass PR tatsächlich die Linke und das Konzept der Umverteilung stärkt. Wir behaupten daher, dass die allgemein akzeptierte Sichtweise historisch, analytisch und empirisch falsch ist. Unsere Erklärung für die Einführung der PR ist eine grundlegend andere: Durch die Integration zweier gegensätzlicher Interpretationen von PR – das Konzept der minimal erfolgreichen Koalitionen [minimum winning coalition] gegenüber dem Konzept des Konsens – gehen wir davon aus, dass die Rechte PR übernommen hat, als ihre Unterstützung für konsensuelle rechtliche Rahmenbedingungen (besonders im Arbeitsmarkt und in der Ausbildung neuer Arbeitskräfte, wo spezifische Investitionen wichtig waren) wichtiger wurde als ihre Abneigung gegen die Umverteilungsauswirkungen; dies passierte in den Ländern, die vorher eine eng organisierte kommunale Wirtschaft hatten. In Ländern mit relativ schlechten Arbeitgeber- Arbeitnehmer-Beziehungen und einer schwach ausgeprägten Koordination zwischen Wirtschaft und Gewerkschaften hatte die Beibehaltung von Mehrheitssystemen die Funktion, die Linke in Schach zu halten. Diese Tatsache erklärt die enge Beziehung zwischen den bestehenden Varianten von Kapitalismus und Wahlsystemen und warum diese weiterhin fortbestehen. -- The standard explanation for the choice of electoral institutions, building on Rokkan’s seminal work, is that proportional representation (PR) was adopted by a divided right to defend its class interests against a rising left. But new evidence shows that PR strengthens the left and redistribution, and we argue the standard view is wrong historically, analytically, and empirically. We offer a radically different explanation. Integrating two opposed interpretations of PR – minimum winning coalitions versus consensus – we propose that the right adopted PR when their support for consensual regulatory frameworks, especially of labor markets and skill formation where co-specific investments were important, outweighed their opposition to the redistributive consequences; this occurred in countries with previously densely organized local economies. In countries with adversarial industrial relations, and weak coordination of business and unions, keeping majoritarian institutions helped contain the left. This explains the close association between current varieties of capitalism and electoral institutions, and why they persist over time.Economic Models of Political Processes,Government,War,Law,and Regulation (Comparative),Political Economy of Capitalism

The Living Standards, Incomes and Accommodation Costs of Older New Zealanders Revisited

In 2001 the then Ministry of Social Policy published a comprehensive study of the living standards of older New Zealanders. The current paper revisits the estimated relationship between material well-being and the current income and accommodation costs of older people, and questions the extent to which income and accommodation costs directly affect well-being or proxy for other factors. We first extend the estimated relationship between material well-being and current income to include the source of the income as well as its level. We find that controlling for different income sources roughly halves the estimated associative effect of income on material well-being. Furthermore, for a given level of income, those with higher fractions of either employment earnings or capital investment income have significantly higher material well-being scores, while those with a higher fraction of income from benefit allowances have lower scores. One interpretation is that these factors may proxy for other causal factors, such as health and wealth effects, rather than reflecting a direct income effect. Next, we extend the original specification between material well-being and accommodation costs to, first, include property rates as an accommodation cost and, second, to control for the type of housing tenure (freehold homeowner, mortgaged, renter, and no accommodation costs). We find that, controlling for housing tenure, the estimated effect of accommodation costs (including rates) is, at most, half that originally estimated and, for some specifications, insignificantly different from zero. Furthermore, controlling for the level of accommodation costs, mortgage holders and renters have significantly lower material well-being scores than freehold homeowners. These findings suggest that understanding what influences the material well-being outcomes of older people is not as straightforward as might be suggested by the simple association of certain variables.Living standards; material well-being; current income; accommodation costs