Message ordering based on the strength of a causal relation

Abstract

In distributed computing, nondeterminism of execution is inevitable because of asynchronous concurrent execution and the unpredictable message transmission delay. We propose a message ordering method, which is called extended causal ordering; to control the execution order which reflects the programmers' intention in object-oriented distributed computing. To do this, we introduce the strength of causal relation. Extended causal ordering is determined according to strength of causal relation. The strength of causal relation between two messages are decided based on the intention of a programmer in a program. Moreover, we define the representation of a logical time, which express an extended causal order: each message has its time stamp which has a logical time as a value. An object can determine the acceptance order of the message according to the value of its time stamp. We adopt the roll-back mechanism as an algorithm which control the acceptance order of the received messages. In this algorithm if the time stamp of the received message is smaller than current time of the object, the object undo the operations that the object did. Extended causal ordering is useful in an object-oriented distributed system, where a programmer intend to control the order of accepting messages.