Planning under Uncertainty in Dynamic Domains - Carnegie Mellon ...

2.2. Reactive planning and approaches that mix planning with execution. 11ronment and chooses an action based on this information, similar to a policy ona Markov decision process described in Section 2.4. These systems have no explicitrepresentation of a global plan and some, such as Brook's subsumption architecture[Brooks 1986] and Agre and Chapman's Pengi [Agre & Chapman 1987],have little or no internal state. The subsumption architecture arranges reactive subsytemsinto hierarchies, so that higher-level behaviour is achieved by one systemover-riding, or subsuming, a more basic system. Neither of these systems, however,shows how a reactive plan could be built from a declarative description ofthe environment. Schoppers [Schoppers 1989a] develops a similar scheme in his\universal plans" and describes how they can be created automatically. Other reactiveplanning systems such asprs [George & Lansky 1987], rap [Firby 1987;1989] and hap [Loyall & Bates 1991] do include internal state | prs represents theagent's beliefs, desires and intentions explicitly | and also control structures such asiteration.These systems can produce appropriate behaviour under dierent execution conditionsbut for each contingency that arises the appropriate response must be programmedby ahuman in advance. In many domains, however, it is not feasible tospecify the correct action in each possible situation in advance. Instead we would liketo combine the speed of a reactive planner in familiar situations with the exibilityof a classical planner in situations not previously anticipated. Systems have beendeveloped for this purpose that combine reactive execution with classical planning,using the latter when no pre-programmed response is available for some contingency.Both the Theo-agent [Blythe & Mitchell 1989] and the \anytime synthetic projection"technique of Drummond and Bresina [Drummond & Bresina 1990] followreactive rules if they are present, and otherwise fall back on planning and then compilethe resulting plan into new reactive rules to be used in future episodes. The twosystems dier on the action and goal representation and on the planning technique.The Theo-agent uses a strips-like action representation and backward chaining. Inanytime synthetic projection, forward chaining is used with a richer action and goallanguage that can specify probabilistic outcomes, exogenous events and goals to maintainpredicates over time intervals.A mixed planning and execution strategy can provide further advantages if it isunder explicit control. Some goals can be planned for in advance, while others can bedeferred until part way through the execution, when there may be more informationabout the best course of action. By not always giving priority to reactive rules, themixed-strategy system can avoid pitfalls in some cases. On the other hand, delayingplanning for these goals can drastically reduce the number of alternatives to consider.Gervasio shows how to build \completable plans" which are designed to be incomplete,and amenable to being further elaborated during execution [Gervasio & DeJong1994; Gervasio 1996]. Goodwin [Goodwin 1994] considers the question of when toswitch from planning to executing in a time-dependent planning problem. Onder andPollack [Onder & Pollack 1997] describe a probabilistic planner that reasons aboutwhich contingencies to plan for before execution and which to defer. Washington