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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