消息日志方法中的不确定性问题

NONDETERMINISM IN MESSAGE LOGGING

消息日志是用于多进程、分布式系统中状态恢复的一种方法．本文针对传统的消息日志方法仅仅适用于确定性进程的局限性，提出一种新的消息日志思想，充分考虑到不确定性的存在在容错方面的积极作用，主张在满足应用进程一致性语义的基础上，在一定程度上允许不确定性现象的存在．从而以新的角度看待单一进程和分布式并发系统中存在的不确定性所带来的状态重建不能完全复原的问题．这样，消息日志亦能适用于某些不满足确定性条件的进程．

Message logging protocols are a common method of state recovery in multiprocess distributed systems and it has many advantages that can not be replaced by the method of checkpointing. But the assumption of piecewise deterministic execution model (PWD) for process in message logging is hardly satisfied in many system instances. In this paper, a new method of message logging is proposed in which the active effect of non-deterministic execution to fault-tolerance is considered and the phenomenon of non-determinism is permitted in process (or system) replay to some extent. The authors point out that cornplete state recovery is not necessary and the state diversity caused by non-determinism maybe the bridge to bypass the fault. The concept of strict replay and consistent replay is described so another way is given to deal with the traditional assumption of process in message logging approach. Single process and distributed concurrent system are studied respectively. A simple prot0type is surveyed in the end of the paper. The authors hope that the method proposed in this paper could give a more widely area for applications of message logging.