Pheme: Synchronizing Replicas In Diverse Environments

This paper presents Pheme1, a peer-to-peer data synchronization protocol that can be used to construct new distributed file systems that share data across collections of devices with limited, varying, or intermittent connectivity. The suitability of Pheme for such environments is not matched by existing protocols. With current technology trends, devices will almost-always be connected to a server or to another device, synchronizing different sets of data as they please. Existing protocols often assume a coarse-grained synchronization model which makes changing the data set of an already established synchronization stream expensive. Pheme introduces fine-grained synchronization so that devices can cheaply change the data set they want to synchronize while they are connected to the other device. In addition, it uses a new dependency summary vector (DSV) scheme to detect conflicts with no extra overhead despite network interruptions and exposes flexible commit mechanisms to allow applications to implement their own commit schemes. Because Pheme provides developers with various synchronization options, systems can be built that send the right data via the right network paths and dramatically outperform traditional client-server or server-replication protocols. At the same time, Pheme is more efficient than existing similarly flexible protocols like PRACTI – we observe several orders of magnitude of bandwidth savings in some experiments.