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 stud): 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 nines availability at a cost of six times the storage footprint of the data collection when the average peer availability is only 24%.