Composite Programs: Hierarchical Construction, Circularity, and Deadlocks

This paper considers a graph-oriented, nonprocedural, development environment where new composite programs are constructed by coupling a collection of existing component programs, each of whose interface is defined by a fixed number of input ports and output ports. When the coupling graph is cyclic there is the possibility of a deadlock. The intent of this paper is to present a system that permits hierarchical construction of programs, while testing (via a simple algebraic procedure) the resulting composite programs for communication deadlocks. Specifically, the paper: 1) describes a decomposition-based approach to cycle enumeration; 2) presents a formal, graph-theoretic, model of communication behavior for a class of atomic programs. The model is then used to derive necessary and sufficient conditions for a deadlock to arise in a cycle; and 3) describes techniques for dealing with a special class of deadly cycles, once identified.