Hi Huseyin,
I do not want Time being an actor in order to satisfy some schoolmaster
telling me that all use cases must have actors. I want Time being an actor
so that I can easily collect use cases, which are initiated at specific
moments in time. I need this for reports etc. I would be quite as happy
having Time being a trigger source, or whatever name one could invent.
What is a trigger and how do you model it in UML?
"Outside temperature goes above 10 degrees C" is an event in the real world.
The system might be polling a temperature sensor in order to catch this
event, or the sensor might be external to the system and send a message to
the system, when the sensor catches the event. In both situations there is a
source (time/sensor) to the event. The original phrase is probably precise
enough to define the general behaviour of a system, but you might end up in
discussions with your customer, say if the system would only poll the sensor
once a day. The source to the "Outside temperature goes above 10 degrees C"
event could be named Environment (I fear that this is a basis for an
extension of the discussion :-) ).
I agree to your red herring, but what is the use of introspective use cases,
if no one will notice them being there at all?
Lars.
-----Original Message-----
From: Angay, Huseyin (Huseyin)** CTR ** [mailto:[EMAIL PROTECTED]]
Sent: 27. marts 2001 11:29
To: '[EMAIL PROTECTED]'
Subject: Re: (ROSE) Use cases: When its not clear who gets the value
The earlier suggestion from Eric of simply specifying that a use case
happens regularly every day at 10:00, say, is generally sufficient. You
could make this the first line of the use case, or, as I normally recommend,
put it in the Trigger field.
Introducing Time as an actor is a kneejerk reaction based on earlier rules
of thumb taught to budding use case writers: "All use cases have actors."
This is fine for most use cases that you write. Unfortunately, what is good
enough when taught in three day courses is not good enough for the rest of
your career. The actual rule should be phrased, "Be extremely suspicious of
use cases without actors." That doesn't mean that you should dismiss them or
invent actors for the sake of making your use case compliant. It means that
a use case without an actor should prompt you to look again. If the use case
is acceptable otherwise, forget the actor.
(By the way, triggers do not have to have actors. They may just happen.
"Outside temperature goes above 10 degrees C" or "10:00 o'clock daily" are
good triggers; and for systems that do not rely on outside temperature
sensors or clocks, no actors are needed to do the prompting.)
Another red herring worth binning:
Actors do not need to get any benefit from a use case.
BUT, a use case must have Stakeholders who get a benefit from the correct
execution of the use case.
Quite often, the stakeholders may not even appear in the list of actors of a
system. This does not make them any less involved. They are just not
~directly~ involved.
In summary, if you can find a stakeholder for your use case, you can
dispense with actors if the system can internally trigger the execution of
the use case. Some use cases are even purely introspective, in that there
may be no immediate outside visibility of their execution.
Regards,
Huseyin Angay
Karabash Ltd.
www.karabash.co.uk
> ----------
> From: Eric D. Tarkington[SMTP:[EMAIL PROTECTED]]
> Reply To: Eric D. Tarkington
> Sent: 27 March 2001 7:31
> To: Rose Forum (E-mail)
> Subject: Re: (ROSE) Use cases: When its not clear who gets the value
>
>
> Lars and others,
>
> When it's a requirement that a system acts "independently" on the basis
> of time, it is unnecessary, but probably harmless, to use Time as an
> actor. What you're doing in that case is putting an aspect of the
> platform outside the system boundary, which is within the scope of your
> authority as analyst (thank-you, Les Munday).
>
> I don't think modelling Time as an actor does much good in ordinary
> cases, but it's cool (which may be enough motivation to do it).
>
> As to communicating with customers, there may be less here than meets
> the eye. Because of constraints on your time and effort, there are
> numerous cases in which you just have to tell a customer "at this point,
> a miracle happens", or give a Startrek engineer's explanation that
> passifies 'em. I'm not knocking this approach very much -- it may be a
> practical and effective substitute for knowledge when the user doesn't
> need to know the real details.
>
> As to communicating with designers, for some of them you can just use
> "communicating with customers", above. For the grumpy designers that
> want more, or actually get to implement the timely behaviour, you should
> avoid letting them force such implementation details into high-level
> models of the system. Unless it's really cool, of course.
>
> This is all a matter of opinion, for now. When use case diagrams or
> similar models can be used to generate executable systems automatically,
> issues like this will meet a definitive test. Until then, you can
> Rational-ize doing what you please.
>
> Finally, in answer to Lars' PS, a riddle (probably quoted more or less
> correctly) from J. R. R. Tolkein's THE HOBBIT:
>
> "This thing all things devours;
> Birds, beasts, trees, flowers;
> Gnaws iron, bites steel;
> Grinds hard stones to meal;
> Slays kings, ruins town,
> And beats high mountains down."
>
> -Eric :?)
>
> Lars Hauschultz wrote:
> >
> > Hi Eric and others,
> >
> > I agree that you don't have to model everything. But what if it is an
> > important feature of the System that it from time to time does something
> on
> > its own? How do you communicate this feature to the customer and the
> > designers, if you do not include it in your model? If you include it and
> you
> > then have to write a number of use cases, which start with "This use
> case is
> > initiated automatically every ...", wouldn't it be convenient to have
> Time
> > as an Actor to hold all the strings to these use cases in its hand, even
> if
> > Time does not get a value from doing so?
> >
> > Lars Hauschultz
> >
> > P.S.: How do you actually know that Time does not get a value from this?
> Who
> > knows Time that well? ;-)
> >
> > -----Original Message-----
> > From: Eric D. Tarkington [mailto:[EMAIL PROTECTED]]
> > Sent: 22. marts 2001 19:33
> > To: [EMAIL PROTECTED]
> > Subject: Re: (ROSE) Use cases: When its not clear who gets the value
> >
> > We had another thread on similar issues a couple of weeks ago, and
> > nobody suggested the simplest solution: don't model time or trigger as
> > an actor.
> >
> > You don't have to model everything.
> >
> > In this example, there will be a use case that wakes up "mysteriously"
> > at the right time, whenever that may be. Use of a call to the operating
> > system is an implementation detail that we don't need to model in a use
> > case diagram -- it's a "how", and we're at the "what" level.
> >
> > In documenting the timely use case, you can start the behaviour with
> > "This use case starts when the timer times out." A few more adjectives
> > wouldn't hurt. Maybe the timer issues a more informative command.
> >
> > On the other hand, peoply find it nifty to make Time an actor, and I
> > don't see the harm. The system can talk to Time (by some protocol
> > hidden in the implementation) to say "Wake me up at 8:00am," and Time
> > can talk to the system at the appropriate moment.
> >
> > Obviously, Time gets no value from the use case, other than fulfillment
> > of it's mysterious compulsion to say "beep" after some
> > externally-supplied interval. This is sorta like a dog fetching a
> > stick: you are the system, you selected and threw the stick in
> > anticipation of the dog's reaction, and the dog looks an awful lot like
> > an actor when he brings it back.
> >
> > Don't worry, be happy.
> >
> > -Eric
> >
> > "Frank I. Reiter" wrote:
> > >
> > > Thanks to all those who have replied so far.
> > >
> > > A couple people have asked, in one way or another, "Who is it that
> > ultimately gets value from this use case?"
> > >
> > > What the use case accomplishes is that the information is the system's
> > database is more up to date after each execution than immediately
> before.
> > Who cares about that? Several actors who get that value through other
> use
> > cases in which they interact with the information that this use case
> > updates.
> > >
> > > They do not however participate in this interaction between my system
> and
> > an external system, and may not in fact be aware that it even happens.
> > >
> > > Since they do not initiate, participate in, or know about this use
> case, I
> > have trouble thinking of them as actors.
> > >
> > > Frank.
> > ************************************************************************
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