message/uml 4

2.2 Main MESSAGE Concepts
2.2.1 Foundations
MESSAGE takes UML as a starting point and adds entity and relationship concepts
required for agent-oriented modelling.Agent-oriented modelling borrows from the
study of human organisations and societies in describing the way in which agents in a
Multi-Agent System work together to achieve a collective purpose,and from artificial
intelligence(AI)and cognitive psychology to describe the agents themselves.These
additional concepts can be defined in terms of object-oriented ones,but deal with
ideas and structures at a higher conceptual level.In AI this higher level is often
referred to as“the knowledge level”,contrasting knowledge with data.Essentially,
MESSAGE uses standard UML as its“data level”modelling language,but provides
additional“knowledge level”concepts.These additional concepts are defined in the
MESSAGE metamodel[5].The metamodel also gives a declarative interpretation to
some UML concepts used to describe behaviour.The most significant of these is
“State”which is described hereafter.
The MESSAGE interpretation of State can be described as follows.A UML model
is a collection of objects.A full description of this model at a point in time consists of
a description of the value of every attribute of every object.Let us call such
description a micro-state.It is rarely practical or useful to work directly with micro-
states,however.A State is characterised by a partial description of the model,i.e.a
constraint restricting the micro-state of model to being one of a set of possible micro-
states.The simplest form of constraint would be to give the value of one attribute of
one object in the model.Because States are sets(of micro-states),the language of
Boolean algebra can be used to describe their relationships(set union,intersection and
containment are equivalent to logical or,and implication).
Note that this is entirely consistent with the UML State concept.From the UML
1.3 specification[1]:
A state is an abstract metaclass that models a situation during which some(usually
implicit)invariant condition holds.The invariant may represent a static situation
such as an object waiting for some external event to occur.However,it can also
model dynamic conditions such as the process of performing some activity(i.e.,the
model element under consideration enters the state when the activity commences
and leaves it as soon as the activity is completed).
The rest of this section describes the knowledge level concepts that feature most
prominently in the MESSAGE methodology as it stands at the moment,particularly
those that appear explicitly in diagrams.
2.2.2 Knowledge-level concepts
Most of the MESSAGE knowledge level entity concepts fall into the main categories:
ConcreteEntity,Activity,and MentalStateEntity.The main types of ConcreteEntity
are:
Agent:An Agent is an atomic autonomous entity that is capable of performing some
(potentially)useful function.The functional capability is captured as the agent's