Choosing Partners Based On Availability In P2p Networks

Availability of applications or devices is known to be one of the most critical variables impacting the performances of software systems. We study in this article the problem of finding peers matching a given availability pattern in a peer-to-peer (P2P) system. Motivated by practical examples, we specify two formal problems of availability matching that arise in real applications: disconnection matching, where peers look for partners expected to disconnect at the same time, and presence matching, where peers look for partners expected to be online simultaneously in the future. As a scalable and inexpensive solution, we propose to use epidemic protocols for topology management; we provide corresponding metrics for both matching problems. We evaluated this solution by simulating two P2P applications, task scheduling and file storage, over a new trace of the eDonkey network, the largest one with availability information. We first proved the existence of regularity patterns in the sessions of 14M peers over 27 days. We also showed that, using only 7 days of history, a simple predictor could select predictable peers and successfully predicted their online periods for the next week. Finally, simulations showed that our simple solution provided good partners fast enough to match the needs of both applications, and that consequently, these applications performed as efficiently at a much lower cost. This solution is purely distributed as it does not rely on any central server or oracle to operate. We believe that this work will be useful for many P2P applications for which it has been shown that choosing good partners, based on their availability, drastically improves their performance and stability.