A scalable replica selection strategy based on flexible contracts

The effectiveness of a replica selection strategy depends on its ability to take into account the metrics that really contribute to the improvement of the QoS perceived by the users. These metrics depend mainly on the replicated application semantics and on the host and network resources that the clients own. This paper presents a replica selection strategy that adapts its criteria dynamically so as to satisfy application providers' and clients' requirements, aggregated into an application binding contract and a client binding contract respectively. We also propose simple access performance heuristics which guide the default selection criteria when a binding contract is not required or is impossible to evaluate. Our protocol also aims to achieve scalability, by using the concept of logical domain, which represents the reduced scope for replica selection.