Significantly ordered delivery of messages in group communication

In distributed applications, a group of multiple objects are cooperated. The objects receive messages and then send back the responses while the objects may exchange data messages. Kinds of group communication protocols support the reliable and ordered delivery of messages at the network level. Only messages to be ordered at the application level, not necessarily all messages, are required, to be casually delivered. In this paper, we would like to define the significant order of messages based on the conflicting relation among the requests. We would like to discuss a protocol which supports the significant delivery of messages.