Autonomous replication for high availability in unstructured P2P systems

We consider the problem of increasing the availability of shared data in peer-to-peer systems. In particular, we conservatively estimate the amount of excess storage required to achieve a practical availability of 99.9% by studying a decentralized algorithm that only depends on a modest amount of loosely synchronized global state. Our algorithm uses randomized decisions extensively together with a novel application of an erasure code to tolerate autonomous peer actions as well as staleness in the loosely synchronized global state. We study the behavior of this algorithm in three distinct environments modeled on previously reported measurements. We show that while peers act autonomously, the community as a whole will reach a stable configuration. We also show that space is used fairly and efficiently, delivering three times availability at a cost of six times the storage footprint of the data collection when the average peer availability is only 24%.