Modeling Potential Service Groups

["Wilson, Kirk D" <Kirk.Wilson@ca.com>]

All,

Just an additional thought on the problems we were discussing on the use
of Service Groups to model various "real" things like printers and
shopping carts:

While the discussion focused on the difference between collections by
value and collections by reference, there's another dimension to the
issue (of collections by reference) that may be being ignored in the
Service Group specification but which inevitably enters into the
discussion.  And that dimension is the semantics of the membership
(part/whole) relationship itself.  UML 1.0 had struggled with this
problem (some would perhaps say, not too successfully) in its
distinction between aggregation and composition.  But the same issues
came up in the conference call.  For example, the relationship between a
printer and its jobs might be modeled in UML 1.0 as composition because
of the lifetime dependency implications of the relationship.  Arbitrary
containership, which characterizes the relationship between a shopping
cart and its contents, would constitute yet a third semantics (which UML
apparently did not feel was important enough to provide built-in
modeling).

It seems to me, in just reading the spec, that the intended relationship
is rather loose.  Which implies that we can't expect it to model every
(any - ??) part/whole relationship with accuracy.  And it may be
necessary to explicitly acknowledge the semantics "mismatch" in things
that are modeled as Service Groups.  We need to answer the questions:
"What is the expected semantics of the SVG membership relationship?"
and "How do real world relationships differ from it?"

Kirk Wilson
Office of the CTO
603 823 4023