Reactive planning

In artificial intelligence, reactive planning denotes a group of techniques for action selection by autonomous agents. These techniques differ from classical planning in two aspects. First, they operate in a timely fashion and hence can cope with highly dynamic and unpredictable environments. Second, they compute just one next action in every instant, based on the current context. Reactive planners often (but not always) exploit reactive plans, which are stored structures describing the agent's priorities and behaviour. The term reactive planning goes back to at least 1988, and is synonymous with the more modern term dynamic planning.

There are several ways to represent a reactive plan. All require a basic representational unit and a means to compose these units into plans.

Finite state machine (FSM) is model of behaviour of a system. FSMs are used widely in computer science. Modeling behaviour of agents is only one of their possible applications. A typical FSM, when used for describing behaviour of an agent, consists of a set of states and transitions between these states. The transitions are actually condition action rules. In every instant, just one state of the FSM is active, and its transitions are evaluated. If a transition is taken it activates another state. That means, in general transitions are the rules in the following form: if condition then activate new state. But transitions can also connect to the 'self' state in some systems, to allow execution of transition actions without actually changing the state. There are two ways of how to produce behaviour by a FSM. They depend on what is associated with the states by a designer they can be either 'acts', or scripts. An 'act' is an atomic action that should be performed by the agent if its FSM is the given state. This action is performed in every time step then. However, more often is the latter case. Here, every state is associated with a script, which describes a sequence of actions that the agent has to perform if its FSM is in a given state. If a transition activates a new state, the former script is simply interrupted, and the new one is started. If a script is more complicated, it can be broken down to several scripts and a hierarchical FSM can be exploited. In such an automaton, every state can contain substates. Only the states at the atomic level are associated with a script (which is not complicated) or an atomic action. Computationally, hierarchical FSMs are equivalent to FSMs. That means that each hierarchical FSM can be converted to a classical FSM. However, hierarchical approaches facilitate designs better. See the paper of Damian Isla (2005) for an example of ASM of computer game bots, which uses hierarchical FSMs.

Both if then rules and FSMs can be combined with fuzzy logic. The conditions, states and actions are no more boolean or "yes/no" respectively but are approximate and smooth. Consequently, resulted behaviour will transition smoother, especially in the case of transitions between two tasks. However, evaluation of the fuzzy conditions is much slower than evaluation of their crisp counterparts. See the architecture of Alex Champandard.

Reactive plans can be expressed also by connectionist networks like artificial neural networks or free flow hierarchies. The basic representational unit is a unit with several input links that feed the unit with "an abstract activity" and output links that propagate the activity to following units. Each unit itself works as the activity transducer. Typically, the units are connected in a layered structure. Positives of connectionist networks is, first, that the resulted behaviour is more smooth than behaviour produced by crisp if then rules and FSMs, second, the networks are often adaptive, and third, mechanism of inhibition can be used and hence, behaviour can be also described proscriptively (by means of rules one can describe behaviour only prescriptively). However, the methods have also several flaws. First, for a designer, it is much more complicated to describe behaviour by a network comparing with if then rules. Second, only relatively simple behaviour can be described, especially if adaptive feature is to be exploited.

Typical reactive planning algorithm just evaluates if then rules or computes the state of a connectionist network. However, some algorithms have special features. Rete evaluation: with a proper logic representation (which is suitable only for crisp rules), the rules need not to be re evaluated at every time step. Instead, a form of a cache storing the evaluation from the previous step can be used. Scripting languages: Sometimes, the rules or FSMs are directly the primitives of an architecture (e. g. in Soar). But more often, reactive plans are programmed in a scripting language, where the rules are only one of the primitives (like in JAM or ABL).