ABSTRACT
Represent a distributed system by a graph G = (V, E), where V is a set of nodes and E is a set of edges joining pairs of nodes. Each node is a sequential process, and each edge corresponds to a communication channel between a pair of processes. Unless otherwise stated, we assume the graph to be directed-an edge from node i to node j will be represented by the ordered pair (i, j). An undirected edge between a pair of processes i, j is equivalent to a
pair of directed edges, one from i to j and the other from j to i. e actions by a node can be divided into the following four classes:
1. Internal action: An action is an internal action when a process performs computations in its own address space resulting in the modication of one or more of its local variables.
