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communityUML 2torial
A place is used to represent some region of space in the system.
UML 2 does not provide any theory of place. However, some of the definitions or modeling invariants mention place, for example, those of object, action, and interface.
Q: Why have the concept of place in UML 2?
A: UML to needs to be able to specify distributed systems. It is not possible to do that without some concept of place.
Q: Well, if place is really necessary, why does UML 2 have such a puny concept of place.
A: This is the minimal necessary addition to UML 1, to enable modeling of distributed systems. UML 1 permits the specification of the location of components on nodes, but not the specification of the location of a node.
Q: How do I use place in a UML 2 model?
A: The modeler may declare as many places as are necessary or desirable, and specify the place of objects, actions, and interfaces.
Q: Suppose I need more than this minimal theory of space.
Simply add the necessary definitions and modeling invariants. For example, specify that places have co-ordinates of one kind or another. Or define a function from two places to a distance.
Q: Why do you say 'region of space.'
A: For the same reason we say 'interval of time.' Due to the nature of computer memories, (and anyway, due to the nature of clocks and measuring sticks) the specification of points in space and time is not possible, only approximations, of whatever precision is needed by the application. However, just as with time, a modeler wishing to have points in space may have them in a UML 2 model, by adding the required modeling invariants.
Of course, applications in which place is a primary concept, will have an explicit theory of space.
Q: UML 1 is for distributed systems. It seems to get along without an explicit concept of place.
A: True. See Place in UML 1.
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