RE: [wsrf] Modeling Potential Service Groups

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

Andrea,

Your comment forced me to go back and review the precise semantics of
composition according to UML 1.0.  You may be correct that composition
is not the perfect way to represent the relationship between printer and
jobs, but not for the reason you give.  In a rather curious move in UML
1.0, responsibility for a composite part may be delegated to another
object.  From the UML Reference Manual (p. 227): "If the composite is
destroyed, it must either destroy all its parts *or else give
responsibility for them to other objects*."  (I always thought that was
a little strange, but there it is!)  Actually, the lifetime semantics of
a UML composition and a printer/job relationship may not QUITE match,
but, on the other hand, I'm less sure of what UML dependency I would
characterize the relationship as exemplifying.

My suggestion might be that the SVG membership may need a semantic
qualifier to better express the type of relationship between the group
and its members.  Does that sound as if it would be value in this
situation?  

Kirk Wilson
Office of the CTO
603 823 4023
 
 

-----Original Message-----
From: Andrea Westerinen (andreaw) [mailto:andreaw@cisco.com] 
Sent: Monday, June 06, 2005 6:09 PM
To: Wilson, Kirk D; wsrf@lists.oasis-open.org
Subject: RE: [wsrf] Modeling Potential Service Groups

Kirk, There is another dimension to printers and jobs that is not
appropriately captured by either a component or an aggregation
relationship.  If a printer goes down, then its current execution of a
job is lost - but the job itself could be sent to another printer.
Therefore, I would not have used the component relationship at all, but
perhaps a dependency.

BTW, within Cisco, we have a concept of a Collection with a property
that indicates if it is explicit (by explicit reference) or implicit (by
virtue of execution of a query).  This has worked well for us, so far.

Andrea  

> -----Original Message-----
> From: Wilson, Kirk D [mailto:Kirk.Wilson@ca.com] 
> Sent: Monday, June 06, 2005 1:13 PM
> To: wsrf@lists.oasis-open.org
> Subject: [wsrf] Modeling Potential Service Groups
> 
> 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
>  
>  
>