Propagators and concurrent constraints

Abstract

A constraint is a numeric or geometric relationship between objects. Constraints have a declarative nature and are a natural way for describing relationships between objects. Combining constraint systems and object-oriented programming (COP) appears hard. All existing systems implicitly compromise the encapsulation principle of COP. Also, the constraint resolving mechanism is typically centralized, thereby restricting parallel execution of a constraint program. This paper proposes a mechanism that provides for constraints in the concurrent (parallel) object-oriented Procol environment. Constraints are implemented using propagators. Propagators are triggered by messages that are sent to objects. Propagator constraints are solved by local propagation of known state, intuitively allowing for parallel execution of a Procol constraint program.

Data model for extensible support of explicit relationships in design databases

“…These operations act as triggers that cause the monitor to execute the enforcement rules maintaining the constraints. The syntax and implementation of monitors are derived from the propagator (Laffra and van den Bos, 1991) which demonstrates that inter-object constraints can be implemented in a manner that does not violate encapsulation. By defining monitors as properties of relationship objects, the SORAC data model combines the benefits of relationships as modeling constructs with the encapsulation of the propagator.…”