A Formal Model of Planning for Concurrency

While classical planning is still mostly concerned with sequential plans, numerous realistic domains are inherently concurrent. Especially planning for multi-agent systems necessitates a clear formal concept of plans in such domains. This paper presents a basic semantic model for concurrent plans using actions with arbitrary effects over time and shared accesses to resources. Two complementary forms of event interaction, asynchronous and synchronous concurrency, are formally described that allow to automatically compute complex joint effects of concurrent actions if possible, but also to explicitly specify combined effects that are not derivable from the single events. According to the formal model, a description language for concurrent domains is defined. The language BTPL (Basic Temporal Planning Language) allows also for rich specifications of initial and goal situations including triggered or exogenous events over time. BTPL is downward compatible to PDDL and may therefore be helpful in bringing classical and multi-agent planning closer together.