next up previous
Next: Some Built-in Constraint Agents Up: Active Constraints Previous: Propagating New Information

Constraint Agents

  We have encountered two very different kinds of constraints. Primitive constraints are held in a constraint store, and tested for consistency by a constraint solver. On the other hand propagation constraints actively propagate new information, and they operate independently of each other. Propagation constraints are more commonly called constraint agents. The behaviour of a constraint agent is to propagate information to the underlying store. In case the underlying store is a constraint store, the information propagated is expressed as primitive constraints.

Constraint agents are processes that involve a fixed set of variables. During their lifetime they alternate between suspended and waking states. They are woken from suspension when an extra primitive constraint on one or more of their variables is recorded. Sometimes, after checking certain conditions, the woken agent simply suspends again. Otherwise the agent exhibits some active behaviour which may result in new agents being spawned, new primitive constraints being added to the store, or an inconsistency being detected (which is equivalent to an inconsistent constraint being added to the store). Subsequently the agent either suspends again, or exits, according to its specification.

Mark Wallace
Wed Sep 3 18:36:40 BST 1997