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Subject: 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
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