Loose synchronization for large-scale networked systems

Traditionally, synchronization barriers ensure that no cooperating process advances beyond a specified point until all processes have reached that point. In heterogeneous large-scale distributed computing environments, with unreliable network links and machines that may become overloaded and unresponsive, traditional barrier semantics are too strict to be effective for a range of emerging applications. In this paper, we explore several relaxations, and introduce a partial barrier, a synchronization primitive designed to enhance liveness in loosely coupled networked systems. Partial barriers are robust to variable network conditions; rather than attempting to hide the asynchrony inherent to wide-area settings, they enable appropriate application-level responses. We evaluate the improved performance of partial barriers by integrating them into three publicly available distributed applications running across PlanetLab. Further, we show how partial barriers simplify a re-implementation of MapReduce that targets wide-area environments.