Three Mechanisms for Managing Resource Constraints in a Library for Constraint-based Scheduling*

ILOG SCHEDULE is a C++ library aimed at simplifying the development of industrial scheduling applications. SCHEDULE is based on SOLVER, a generic tool for object-oriented constraint programming. SCHEDULE includes three categories of predefined constraints: temporal constraints, capacity constraints, and resource utilization constraints. Three distinct mechanisms are available to deal with resource utilization constraints. The first mechanism relies on explicit time-tables to maintain information about the variations of resource utilization and resource availability over time. The second mechanism relies on a generic disjunctive constraint which ensures that incompatible activities (for example, activities which require a common resource of capacity 1) cannot overlap in time. The third mechanism, known as edge-finding, considers arbitrary tuples {A1 ... An} of activities to prove that some activity Ai must execute first, or must execute last, among {A1 ... An}. The edge-finding mechanism is a priori more CPU-time consuming than the two other mechanisms, but results in the assignment of more precise earliest and latest start and end times to activities. Each of the three mechanisms is useful, as the time-table and disjunctive mechanisms enable the expression of wider classes of constraints, while edge-finding is in general the most effective in pruning the search space.