Re: activator association in a Sequence Diagram



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From: Alex Bagehot (A.S.P.Bagehot@hw.ac.uk)
Date: Wed 04 Sep 2002 - 15:52:52 BST


Hi Simona,

As far as I understand it:

message sequencing in a SD (sequence diagram) or CD (collaboration 
diagram) is specified by the UML Message class.

The activator association of the Message class in a SD or CD tells a 
message which other message activated it.

if you have a message coming into the top of an activation bar in an 
SD, all of the outgoing messages (on that activation bar) are 
activated by the incoming one. [ to make the message sequence 
complete, outgoing messages on an activation bar are associated 
together with the predecessor Message association, like a linked 
list.]

> 1- each message belonging to an interaction has got an
> activator message,
> except for the initial messages (page 2-134)

right. the first message in a sequence(in fact all the messages in the 
first activation bar) isn't activated by anything else, otherwise the 
sequence of Messages in the interaction (diagram) would become cyclic.

> 2- the well-formed rule for Message class:
> "predecessors must have the same activator as the message"
> self.allPredecessors->forAll (p|p.activator = self.activator)

> Since  an initial message belongs to (at
> least)  a set of type "m.allPredecessors" where m is a
> not-initial message
> belonging to the same interaction, the well-formed rule says
> that
> the initial message has an activator.

right in the sense that the initial message will have an activator 
association. but the reference to that activator will evaluate to null 
(ie. it doesn't have one).

best regards,

Alex Bagehot
Msc I.T. (Systems) Student, Heriot-Watt University.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
On Wed, 4 Sep 2002 15:56:30 +0200 (MET DST), Bernardi Simona 
<bernardi@di.unito.it> wrote:

> 
> Hello,
> 
> can anybody tell me what is  the meaning of "activator"
> association in the
> context of sequence diagrams (if there is one) ?
> 
> The information given in the UML manual (version 1.4) seem to
> me
> contradictory, in particular the following statements:
> 
> 1- each message belonging to an interaction has got an
> activator message,
> except for the initial messages (page 2-134)
> 
> 2- the well-formed rule for Message class:
> "predecessors must have the same activator as the message"
> self.allPredecessors->forAll (p|p.activator = self.activator)
> 
> where self.allPredecessors is the set of all Messages that
> precede the
> current one (i.e., self).
> 
> Since  an initial message belongs to (at
> least)  a set of type "m.allPredecessors" where m is a
> not-initial message
> belonging to the same interaction, the well-formed rule says
> that
> the initial message has an activator.
>    
> Thanks,
> Simona.

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