Klaus,
An actor is someone or something that initiates a use case. The manner of
initiating the use case is a message or event.
If an actor is real, the event might be a keystroke or a mouse click,
whatever. It doesn't matter how, only that an action has occurred, an event
or message visible by the system is visible and trappable.
If the actor is not real, the event or message is still the same.
Actors initiate use cases. They are the beginning. Events do not
activate actors. If they did, actors would serve no purpose.
The passage of time is not an event. An alarm going off at 6am to wake
you up is an event. A loud screeching sound is the message to you to wake
up. The system that monitors time and decides if it is time to generate a
message / event is often referred to as a system clock or timer.
Finally, you seem a little unclear about what a message is. A message is
the means by which objects communicate. When an actor decides to invoke a
use case, he does so by sending a message into the system. Although
different in generation, the result is the same. A message into the system.
Cheers.
-----Original Message-----
From: Oppermann, Klaus [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, February 28, 2001 12:05 AM
To: 'Flum, Michael'; '[EMAIL PROTECTED]'
Cc: '[EMAIL PROTECTED]'
Subject: AW: (ROSE) "System" as Actor
I think a use case is initiated by an actor and the actor becomes active by
an event.
The initiation of an use case is not a message, it is something what the
actor can do with the system, e.g. start a dialog by a human actor. A
possible event is also the elapse of time, which may causes the initiation
of an use case.
Klaus
-----Ursprüngliche Nachricht-----
Von: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]Im Auftrag von Flum, Michael
Gesendet am: Dienstag, 27. Februar 2001 21:24
An: '[EMAIL PROTECTED]'
Cc: [EMAIL PROTECTED]
Betreff: RE: (ROSE) "System" as Actor
I guess this horse isn't dead yet.
Here's my spin:
One critical feature of an actor is that it sends messages to the system.
Time can't send messages. Time is simply a data source (at best). The
system/pc clock can send messages. They are distinct, trappable events that
the software system can understand.
Time and clock together are their own external system and are
appropriately noted as an actor on UC diagrams.
-----Original Message-----
From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, February 27, 2001 12:04 PM
Cc: [EMAIL PROTECTED]
Subject: RE: (ROSE) "System" as Actor
> Using "Time" instead of "System Clock" as an actor is an interesting
> idea -- sort of an Ingmar Bergman thing. The "grim reaper" has already
> been used for death, but "father time" is very similar -- do you think
> it would be suitable as an icon?
>
Time as an actor comes under both (1) and (2). Both timers and the actual
time start to become important in the design of real-time systems. Yet, I
have found stating that a use case activates/starts every ten minutes or
daily at midnight is enough. Or you could mention that an alternative flow
takes place if some actor (or some included use case) does not finish its
task in 30 microseconds.
As soon as you have time as an actor, you have to start thinking about the
messages we send to Time and the messages that we receive from it. Are the
contents of these messages important? Or are we interested in just
receiving
timely reminders?
[Les]
Just one small point, which is that the only reason I could see a system
sending messages to 'Time' would be if you were trying to build a time
machine.
Could it really be that simple :-)
Les.
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