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UML concepts are used to model MESSAGE entities at a detailed (or micro) level. That is, from a structural standpoint they are objects with attributes and operations realised by methods; from a behavioural point of view they are state machines. The main concepts that are used to define the 'physics' of the MESSAGE world view are: Action, Event and Situation. Action and Event correspond quite closely to UML behavioural concepts of the same name. The world is seen as a collection of StateMachines. A full description of a world state consists of a description of the state of every StateMachine within it at a point in time. The evolution of the world in time can be described fully by a description of the world state at a specific point in time and a list of all the Events in which the world participates before and after that time. This full description is referred to as the world history.
A Situation is a knowledge (or macro) level analogue of a world state, and is similar to the UML concept of State. Except in very simple worlds, it is not useful to work with full state or history descriptions. Many facets of the state will not be relevant to the matter in hand. The concept of Situation is introduced to deal with this. It associates a partial description of the world state and event list (i.e. a constraint on the world history) with a point in time. It signifies that at the specified point in time, the world state is one of the set of states consistent with the constraint.
Temporal relationships: The UML concept of StateTransition is identified with a precedes relationship between entities associated with points in time. The UML mapping connective (bar symbol) and others introduced by AUML [OMG99b] can then be used to form more complex temporal expressions.