What Gregg has been showing here is really interesting, I am not sure
to interpret it correctly, it looks like service or operation
inheritance. I would do it in Java this way:
ShoppingService
SellGoods(Goods)
BuyGoods(Goods)
Goods class
Food class
Hoagies
Meat
Beef
Chicken (well maybe not a good idea at the moment)
Cheese
Swiss
Cheddar
SharpCheddarCheese
MildCheddarCheese
and so forth...
I think there is a fundamental difference between Object Orientation
and Service Orientation there. The problem is not in the Goods
sub-classes tree, I will use composition or extensions instead of
inheritance in XML but that's not the problem, the problem is in the
ShoppingService design.
I got this same kind of discussion already with my technical architects.
When I design an OO API, I will always try to work with abstract types
or interfaces (Goods). That gives me the potential to use polymorphism
and use the same set of operations to manipulate different flavors of
the Goods class. The idea is that I should have no idea of what kind
of object implementation it really is from the moment it fulfills the
interface I will use on top of it (ex: BuyGoods and SellGoods).
In contrast, a service oriented way of thinking would prohibit the use
of abstract types. The service consumer MUST be aware of the concrete
type returned. All the possible attributes and sub-elements must be
exposed in the service interface, only concrete stuff in there. That
means avoiding <ANY> kind of XML types. If you do that, the service
interface is no more self-describing and well-designed I guess.
You could design a service that offers a SellGoods operation and would
accept a list of possible types (Sandwiches, Chips,...) OR design a
set of specialized SellXYZ operations.
In both designs, adding a new type like Chocolates (I am Belgian) will
result in a new service interface. In some cases, I may find it to be
more convenient and less disruptive to just add a specific operation
than changing an existing operation. It does not mean I will start
replicating code within the service implementation, of course.
Am I wrong?
Robin
http://blogs.ittoolbox.com/eai/applications/
--- In [email protected], Gregg Wonderly
<[EMAIL PROTECTED]> wrote:
>
> MonetaryTransaction
> SellSomething
> SellFood
> SellSandwiches
> SellChips
> SellDrinks
> BuySomething
> BuyFood
> BuyHoagies
> BuyMeat
> BuyTurkey
> BuyBeef
> BuyChicken
> BuyCheese
> BuySwissCheese
> BuyCheddarCheese
> BuySharpCheddarCheese
> BuyMildCheddarCheese
>
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