MOVERS-WORLD, and the techniques used to learn cooperative procedures by MOVERS-WORLD agents, is shaped by the fact that communication is the central mechanism for cooperation and coordination. The nature of communication in MOVERS-WORLD is quite different from standard distributed AI planner systems such as [Corkill1979] because communication occurs at run-time rather than as part of the planning process. This is a consequence of our belief that communication takes roughly the same order of time as do primitive actions. Communication between agents in our system is similar to a telephone conversation. Agents do not have to be in the same location to engage in communication. Once a connection is established, communication is limited to a small number of request and response frames. Agents do not use communication to develop any sort of shared structure, such as a SharedPlan [Grosz & Kraus1998].
Two agents are said to `cooperate' if they act or work together to achieve some common purpose and they are `coordinated' when their individual actions are appropriately ordered to support cooperative activity. In our framework, communication is the only mechanism whereby agents can determine if they are coordinated. In other words, there are no global structures (such as blackboards) for agents to use to determine if they happen to be working on the same goal and the agents do not have sufficiently powerful models of each other to use communication-free coordination strategies such as [Huber & Durfee1995]. Each MOVERS-WORLD problem-solving episode includes some goal(s) that can only be solved by coordinated agents, so communication is an essential part of the community activity required to solve problems.
Communication is used to attempt to establish cooperation when the set of agents working on a goal at a given time is inadequate. Cooperation is not guaranteed during communication since agents have their own decision-making strategies1 and even if an agent is willing to cooperate she may be unable to do so. An agent who is unwilling or unable to assist can propose an alternative cooperative activity which the original requester must now contemplate adopting.
Communication and a meta-planner operator jointly form the mechanism whereby coordination occurs in MOVERS-WORLD. When cooperation is first established during communication, the agents must determine how they will coordinate the cooperative activity. Sometimes, nothing need be done - for example, if two lifters are both adjacent to a ready-to-be-lifted box and both are ready to lift it. More often, though, the requester will have to idle for one or several rounds - for example, if the requestee lifter is not currently ready to lift the box. Another common situation involving idling occurs after a lifter agrees to load a box onto a hand-truck for a hand-truck operator. Idling is presently implemented as a busy-wait by pushing a meta-planner operator WAIT onto the plan. While the WAIT is at the top of her plan, an agent is waiting for one of two events to occur: communication indicating that joint action can occur (e.g., the other lifter now indicates she is ready to act) or the completion of her request (e.g. the box appears on the hand-truck). If an agent is idle too long, she will become frustrated and inquire about the status of her request.