BNAIC, Oct, Temporal Plans and Resource Management Pieter Buzing & Cees Witteveen TU Delft

BNAIC, Oct, What about? Temporal planning in multi-agent systems or: distributed issues in planning systems Plan repair (during tactical phase) Introduce resource-based view Use scheduling heuristic Integrate it with multi agent temporal planning

BNAIC, Oct, The Problem Many complex environments need planning and coordination of actions Example: airport, harbor, factory, … Multiple (autonomous) parties involved Issues: conflicting goals, communication, coordination Execution phase is error-prone: Environment is unpredictable, partial information

BNAIC, Oct, Example: Airport

BNAIC, Oct, Relationship MAS and Planning Multi agent view on planning: First understand organizational structure, then think about system performance Isn’t planning already solved? (old AI anyway) The planning thing is a minor (technical) problem; communication, negotiation: those are real challenges! Planning view on multi agent: “Why would you want distributed planners? Centralized solutions are always faster and better” “Why focus on plan repair? The worst-case complexity of re- planning is equal to the original planning problem” How to verify optimality?

BNAIC, Oct, Solution Requirements Respect individual planning tools: Abstract temporal plan model Expect aberrations during plan executions: Flexibility encoded in plan Respect and use agents’ intelligence: Smart coordination (negotiation), no swarm intelligence

BNAIC, Oct, Simple Temporal Problem (STP) Planning as CSP - Dechter, et.al. (1991) Temporal constraints between time point variables Path consistency = arc consistency = polynomial time: O(n^3) Extract schedule is simple (read all lower bounds) Flexibility is maintained

BNAIC, Oct, STP + Preferences Duration action A is [10, 40]: hard constraint In practice: “A takes about 25 minutes, perhaps bit more/less” “25 would be ideal, but [15-30] is okay” Soft constraint expressed as preference function Repair (B&W, 2004a)

BNAIC, Oct, Choices in STPs “either action A before B or action B before A” Disjunctive Temporal Problem MA voting protocol for decision making (B&W, 2004b) Reordering actions as a means of plan repair

BNAIC, Oct, STPs and Resources Planning = action ordering Scheduling = resource assignment Practical planning problems are mix of both… Airport: gates, runways, taxiways Example: 4 flights scheduled on 2 gates

BNAIC, Oct, Scheduling Heuristic for STPs Known scheduling heuristic: flexibility Amount of slack Planning phase: assign action a to resource s.t. flex(a) is max Plan repair: Choose action a s.t. flex(a) is min Move a to resource s.t. flex(a)’ becomes max

BNAIC, Oct, Example (Gate Scheduling) Aircraft has delay: can dock not before t=50 Inconsistency since flex value is negative Find gate with highest flex: g2 Move aircraft to that gate

BNAIC, Oct, Conclusion Trying to bring together: Multi agent system (temporal) Planning Scheduling aspects Application: Collaborating with NLR (National Aerospace Laboratory) Extending airport simulator with MA tools