> ----------
> From: Colin Gourlay[SMTP:[EMAIL PROTECTED]]
> Reply To: Colin Gourlay
> Sent: 27 February 2001 9:11
> To: [EMAIL PROTECTED]
> Subject: Re: (ROSE) "System" as Actor
>
>
> This is an interesting question you pose Eric.
>
> I am quite new to UML and have been doing precisely what you're students
> have been doing. In a separate post, in recent weeks, where I stated
> something to the effect that an actor either gives value to or gets value
> from a use case and went on to say that an actor can be external or
> internal
> to the system under discussion I found that the general consensus of
> opinion
> was that Actors are external and *not* part of the system under discussion
>
The actors and value connection may become a red herring very quickly with
many systems.
It is worth distinguishing stakeholders and actors, the former of which
often get ignored.
The stakeholders expect or provide value to the system, but they don't
always interact with the system.
Actors interact with the system, but they are not always stakeholders. They
may possibly not even get any value from the system.
> That is, both your students and myself appear to have been doing this
> incorrectly.
>
> I accepted what other posters said *but* I would like to pose pretty much
> the same question as yourself but relate it to my own circumstances...
>
> I am writing a middleware application that is to integrate disparate
> systems. The system under discussion has to poll other systems and
> devices
> (these were my external actors) and react accordingly upon receiving
> messages, data or whatever. In this SuD I had an actor (a stereotyped
> class) that was responsible for polling and generally *controlling* the
> coordination/sequence of events of the use cases in the SuD. This actor
> as
> I saw it was very much central (not in a behaviourla sense but in a
> controlling context) to the system being designed.
>
The poller should really not be an actor. Whatever it is polling might be an
actor (in this case, other systems), but the poller itself is part of your
system.
> As Eric says if it "...looks like an actor, walks like an actor, and
> quacks
> like an actor, what is it?"
>
You can dress it up as an actor, but it will give the game away when it
opens its mouth.
Actors are useful to establish the system boundaries. If the actor were part
of the current system, what benefit would you get out of it? You could just
as well replace it with an object on your sequence diagram.
Actors have got one important property: they are *unpredictable*.
Your system is not unpredictable -- at least, it should not be. The use
cases are ine way of enforcing that predictability.
The result of this is that you can model classes with the behaviour that
~you want~, whereas you must model the actors with the assumption that ~they
could do anything~.
> Or is this newbie (me) missing something completely??
>
> Regards
>
> Colin
>
>
Regards,
Huseyin
> ----- Original Message -----
> From: Eric D. Tarkington <[EMAIL PROTECTED]>
> To: ROSE_FORUM <[EMAIL PROTECTED]>
> Sent: Monday, February 26, 2001 9:59 PM
> Subject: (ROSE) "System" as Actor
>
>
> >
> > When drawing their first use case diagrams, my students (including some
> > of my best) sometimes identify the system under development as an actor.
> >
> > I've been telling them not to do this, since my favorite sources don't
> > do it, but lately it has been bothering me.
> >
> > When the system is doing something like scheduling activities for
> > regular or delayed execution, I have seen authorities use a "system
> > clock" actor. Also, in some environments it is quite right to say that
> > the system provides services such as printing. If the system looks like
> > an actor, walks like an actor, and quacks like an actor, what is it?
> >
> > -Eric
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