. Dynamic replication can be used to reduce bandwidth consumption and access latency in high performance "data grids" where users require remote access to large files. Different replication strategies can be defined depending on when, where, and how replicas are created and destroyed. We describe a simulation framework that we have developed to enable comparative studies of alternative dynamic replication strategies. We present preliminary results obtained with this simulator, in which we evaluate the performance of five different replication strategies for three different kinds of access patterns. The data in this scenario is read-only and so there are no consistency issues involved. The simulation results show that significant savings in latency and bandwidth can be obtained if the access patterns contain a small degree of geographical locality. 1

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Replication is a well-known technique to improve the accessibility of Web sites. It generally offers reduced client latencies and increases a site’s availability. However, applying replication techniques is not trivial, and various Content Delivery Networks (CDNs) have been created to facilitate replication for digital content providers. The

As Internet traffic continues to grow and web sites become increasingly coffimplex, performance and scalability are major issues for web sites. Web sites are increasingly relying on dynamic content generation applications to provide web site visitors with dynamic, interactive, and personalized experiences. However, dynamic content generation comes at a cost -- each request requires computation as well as communication across multiple components.

Data replication is used extensively in wide-area distributed systems to achieve low data-access latency. A large number of heuristics have been proposed to perform replica placement. Practical experience indicates that the choice of heuristic makes a big difference in terms of the cost of required infrastructure (e.g., storage capacity and network bandwidth), depending on system topology, workload and performance goals.

This paper introduces a framework for evaluating replica placement algorithms (RPA) for content delivery networks (CDN) as well as RPAs from other fields that might be applicable to current or future CDNs. First, the framework classifies and qualitatively compares RPAs using a generic set of primitives that capture problem definitions and heuristics. Second, it provides estimates for the decision times of RPAs using an analytic model. To achieve accuracy, the model takes into account disk accesses and message sizes, in addition to computational complexity and message numbers that have been considered traditionally. Third, it uses the "goodness" of produced placements to compare RPAs even when they have different problem definitions. Based on these evaluations, we identify open issues and potential areas for future research.

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Replicating Web documents at a worldwide scale can help reduce user-perceived latency and wide-area network traffic. This paper presents the design and implementation of Globule, a platform that allows Web server administrators to organize a decentralized replication service by trading Web hosting resources with each other. Globule automates all aspects of such replication: document replication, selection of the most appropriate replication strategies on a per-document basis, consistency management and transparent redirection of clients to replicas. To facilitate the transition from a non-replicated server to a replicated one, we designed Globule as a module for the Apache Web server. Therefore, converting Web documents should require no more than compiling a new module into Apache and editing a configuration file.

Replication and caching strategies are being used to reduce user perceived delay and wide area network traffic. Numerous such strategies have been proposed to manage replication while maintaining consistency among the replicas. In earlier research, we demonstrated that no single strategy can perform optimal for all documents, and proposed a system where strategies are selected on a per-document basis using trace-driven simulation techniques. In this paper, we demonstrate the need for continuous dynamic adaptation of strategies using experiments conducted on our department Web traces. We also propose two heuristics, Simple and Transition, to perform this dynamic adaptation with reduced simulation cost. In our experiments, we find that Transition heuristic reduces simulation cost by an order of magnitude while maintaining high accuracy in optimal strategy selection.

Dynamic replication can be used to reduce bandwidth and access latency in high performance "data grids" where users require remote access to files that are large: on average, around two Gigabytes. Different replication strategies are defined depending on when, where, and how the replicas are created and destroyed. We describe a simulation framework that we have developed to enable comparative studies of alternative dynamic replication strategies. We present preliminary results obtained with this simulator, in which we evaluate the performance of six different replication strategies for three different kinds of access patterns. The data in this scenario is read-only and so there are no consistency issues involved. The simulation results show that the best strategy has savings in latency and bandwidth of up to sixty percent if the access patterns contain a small degree of geographical locality. 1