Scheduling in a Planning Environment

In a real planning problem, there exists a set of constraints (both temporal constraints and resource usage constraints) which must be satisfied in order to obtain a feasible plan. This requires a scheduling process (after the planning process) which should guarantee the availability of resources and the satisfiability of all the problem constraints. Several approaches have been proposed to deal with planning and scheduling problems. However, these approaches have drawbacks which will be presented here. This paper deals with the main features of a scheduling process in an integrated architecture of planning and scheduling, where both processes work in a simultaneous way. Thus, the executability of each plan is guaranteed as it is being obtained by the planner. The planning process searches among alternative partial plans, where each one of them has its own ordering relations among actions, resource requirements, intermediate states, etc. Since these constraints are provided while the plan is being obtained, the proposed scheduling process should be able to manage them as they are being known. Thus, the scheduler should not obtain a solution after each new asserted constraint but rather it should only maintain the consistency among all the asserted constraints. In addition, the planner keeps track of several alternative open plans, which are suitable for being expanded in each moment. For this reason, the scheduler should maintain the effects of the constraints belonging to different plans that are being explored by the planner. Hence, both specific planning and scheduling optimisation criteria are used in order to improve the behaviour of the integrated system, its efficiency and the quality of the obtained plan.