OpenSolver: A Coordination-Enabled Abstract Branch-and-Prune Tree Search Engine

Coordination programming deals with building complex software systems from largely autonomous component systems. Cooperative constraint solving (see for example [1]), which is widely recognized as a means to improve efficiency of constraint solving, is an area that may benefit from techniques developed in coordination programming, because the solvers that we want to combine are generally autonomous applications that have diverse interfaces. OpenSolver is an experimental constraint solver that has been designed with solver cooperation in mind. As a result, it allows for easy coordination. It implements a branch-and-prune tree search solving algorithm that is abstract in the sense that the actual functionality is determined by software plug-ins in a number of predefined categories, corresponding to different aspects of this solving algorithm. A special category of plug-ins covers the coordination layer of the solver. Through a plug-in in this category, the execution of the solving algorithm can be controlled, and data can be shared with other solvers. The main categories of functional plug-ins are domain types for CSP variables, domain reduction functions (DRF’s) that perform the actual pruning, schedulers that control the application of the DRF’s, branching strategies that expand the search tree, and several categories corresponding to different aspects of a strategy for traversing the search tree. OpenSolver is the basis for implementing DICE (DIstributed Constraint Environment) [2], a framework for cooperative constraint solving where it plays the role of a component solver, and of a wrapper that allows other solvers to be coordinated through it. For this purpose, a coordination layer plug-in is being developed through which OpenSolver instances can participate in a distributed constraint propagation algorithm, and in parallel search. These are the main modes of solver cooperation supported by DICE. Other possible coordination layer plug-ins are a user interface that drives a single OpenSolver as a standalone configurable constraint solver, and interfaces for nested search, through which an OpenSolver can be used as a DRF, or as a branching strategy plug-in.