a b s t r a c tWe study an access trace containing a sample of Wikipedia's traffic over a 107-day period aiming to identify appropriate replication and distribution strategies in a fully decentralized hosting environment. We perform a global analysis of the whole trace, and a detailed analysis of the requests directed to the English edition of Wikipedia. In our study, we classify client requests and examine aspects such as the number of read and save operations, significant load variations and requests for nonexisting pages. We also review proposed decentralized wiki architectures and discuss how they would handle Wikipedia's workload. We conclude that decentralized architectures must focus on applying techniques to efficiently handle read operations while maintaining consistency and dealing with typical issues on decentralized systems such as churn, unbalanced loads and malicious participating nodes.

Peer-to-peer networks based on distributed hash tables (DHTs) have received considerable attention ever since their introduction in 2001. Unfortunately, DHT-based systems have been shown to be notoriously difficult to protect against security attacks. Various reports have been published that discuss or classify general security issues, but so far a comprehensive survey describing the various proposed defenses has been lacking. In this article, we present an overview of techniques reported in the literature for making DHT-based systems resistant to the three most important attacks that can be launched by malicious nodes participating in the DHT: (1) the Sybil attack, (2) the Eclipse attack, and (3) routing and storage attacks. We review the advantages and disadvantages of the proposed solutions and, in doing so, confirm how difficult it is to secure DHT-based systems in an adversarial environment.

Spotify is a peer-assisted music streaming service that has gained worldwide popularity. Apart from providing instant access to over 20 million music tracks, Spotify also enhances its users' music experience by providing various features for social interaction. These are realized by a system using the widely-adopted pub/sub paradigm. In this paper we provide an interesting case study of a hybrid pub/sub system designed for real-time as well as offline notifications for Spotify users. We firstly describe a multitude of use cases where pub/sub is applied. Secondly, we study the design of its pub/sub system used for matching, disseminating and persisting billions of publications every day. Finally, we study pub/sub traffic collected from the production system, derive characterizations of the pub/sub workload, and show some interesting findings and trends.

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Publish/subscribe (pub/sub) is a popular communication paradigm in the design of largescale distributed systems. A provider of a pub/sub service (whether centralized, peer-assisted, or based on a federated organization of cooperatively managed servers) commonly faces a fundamental challenge: given limited resources, how to maximize the satisfaction of subscribers? We provide, to the best of our knowledge, the first formal treatment of this problem by introducing two metrics that capture subscriber satisfaction in the presence of limited resources. This allows us to formulate matters as two new flavors of maximum coverage optimization problems. Unfortunately, both variants of the problem prove to be NP-hard. By subsequently providing formal approximation bounds and heuristics, we show, however, that efficient approximations can be attained. We validate our approach using real-world traces from Spotify and show that our solutions can be executed periodically in real-time in order to adapt to workload variations.