Planning with Respect to an Existing Schedule of Events

Decomposition has proved an effective strategy in planning, with one decomposition-based planner, SGPLAN, exhibiting strong performance in the last two IPCs. By decomposing planning problems into several loosely coupled subproblems, themselves planning problems, a planner can be used to solve each of the subproblems individually. The subplans can then be combined to form a solution to the original problem. When planning for subproblems, it is necessary to account for the interactions between the actions used to solve the current subproblem and the actions chosen to solve other subproblems. The approach taken in SGPLAN is inspiring, but some aspects of the decomposition process are not fully described in the literature. In particular, how subplans are merged to form a complete plan is not discussed anywhere in detail. This paper presents an approach to planning at this subproblem level, detailing how the choices made whilst solving one subproblem can be influenced by the conflicts with other subproblems, and introduces a novel technique employing wait events that can be included in subproblem solution plans to allow the context of the existing schedule to be directly considered.