Causal Order of Application Events is P2P Publish/Subscribe Systems

In distributed systems, a group of multiple processes are cooperating with each other by exchanging messages in networks. A process is considered to be a sequence of events in a finite state machine model. Here, it is significant to decide on which event causally precedes another event for a pair of events in a system. In the famous happened-before relation among events, events are ordered on the basis of send-receive semantics of messages exchanged in networks. In this paper, we discuss a peer-to-peer (P2P) model of a publish/subscribe (P2PPS) system where application events are subscribed, published, and notified by peer process (peers). We discuss the causal dependency of application events in terms of subscription, publication, and notification semantics.