RE: Activity diagram



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From: Chris Britton (chris.britton@blueyonder.co.uk)
Date: Thu 13 May 2004 - 09:40:05 BST


There was a time when many people thought that objects should not be
passed around, only accessed in situ. CORBA for instance, also like UML
from OMG, does not (or at least did not) support the notion of passing
objects. To be more precise, in CORBA an object can move around so long
as the move is invisible to the client program. The rationale was that
objects stood for something in the real world therefore it was best to
have one copy of the object. Furthermore if you believe that an object
has an "identity" then it's hard to associate an identity with a message
being passed from one server to another. If you loose the notion of
object identity then you can't reference the object from other objects.

However in practice it is often a good idea to move objects around. For
instance, an order entry system may create an order object which is
later moved to a manufacturing system and later to a distribution
system.

But note, my second paragraph was talking about objects in a different
context to the first. Distinguish between "business objects" - the
external notion of the object to the end users and "implementation
objects" - the object internal to a computer system. Because internal
objects have an identity, they can't be moved. Thus to move a business
object you must create a message that contains the serialized data of
the object and use the data to create a new object at the other end.

There is nothing stopping an activity diagram being used at either the
business object level (to analyze business processes) or at the
implementation level. When doing analysis at the business object level
you should probably ignore the physical flow of objects around the
system. When doing activity analysis at the implementation object level,
the objects don't move. So in either case, activity diagrams don't need
to show object movement.

I suggest that what's missing in UML is the concept of levels of design
and the ability to show how objects at one level map to objects at
another level. Although I called them implementation objects they too
still map to multiple objects - an object in the database and probably
an object in every tier.

Best regards,
Chris

-----Original Message-----
From: puml-list-request@cs.york.ac.uk
[mailto:puml-list-request@cs.york.ac.uk] On Behalf Of Shashank
Sent: 08 May 2004 10:38
To: UML
Subject: Activity diagram

Is it a standard practice to show the object getting passed between
activities, or is a rarely used .

In other words is it a good practice to do this or not?





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