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> But more importantly, type hierarchy doesn't seem to scale.
> Your payroll example seems to make every service an object.
> Can you take that example further and describe some other
> kinds of objects that might exist? And show the hierarchy?
If you're a bank then you deal with trading partners. That's a type with
many instances, who all, at some level of abstraction, exhibit the same
interface to the bank. The type "trading partner" also has subtypes, for
example foreign exchange dealers, stockbrokers, other banks.
Identifying this type hierarchy in my view can be of real practical use when
it comes to message design. Some of the types might even map reasonably
directly into type definitions in an XML Schema - though that's perhaps
optimistic. But it can certainly help the analysis.
Of course one could argue that we're not using OO concepts here, but rather
the basic ontological methodology on which OO concepts were based. But