Re: signals

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From: Les Munday (
Date: Wed 25 Sep 2002 - 19:16:49 BST

> Hi!
> I am student at UMass, Amherst and I am working with LASER
(Lab for Advanced Software Engineering
> Research). I am working with activity diagrams and tring 
desperately to understand its semantics. We
> are really stuck with signals right now. The UML user guide, 
reference manual give very ambigous
> statemnets about signals. They also say that acticity 
diagrams do not have events but only signals. I
> am not clear on teh exact semantics about signals. Signals 
are also classifiers and we can have a
> class diagram on the hierarchy of signals int he system. Can 
anyone please tell me what exactly
> signals do in an activity diagram and how are they 
I can't help you with the difference between events and signals 
but, my tool shows links between activities as transitions each 
of which is described by an event. Also note that in 
discussions I've had with various people using activity 
diagrams, there are a number of ambiguities in the notation, 
which mean that people are using the same diagram notation to 
mean different things.

Bearing this in mmind, I would suggest that you make you own 
rules, but stay within the UML guidelines. If a word is open to 
interpretation (Signal for example) mnake your own definition 
and stick with it. This is also within the guidelines laid down 
by your tool if it is intelligent enough to understand activity 
diagram notation.

In summary, it's not something I'd lose sleep over, and as I 
just read this morning, the activity diagram notation is going 
to change in UML 2.0 anyway.


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