I'm listening to Pat Helland (a serious thought leader in the service-oriented space) speak and it has me seriously thinking.
I think that one fundamental difference between service-oriented thinking and n-tier is this:
We've spent years trying to make distributed systems look and feel like they run in a single memory space on a single computer. But they DON'T. Service-oriented thinking is all about recognizing that we really are running across multiple systems. It is recognition that you can’t trivialize, abstract or simplify this basic fact. It is acknowledgement that we just can’t figure it out.
Things like distributed transactions that are designed to make many computers look like one. Heck, CSLA .NET is all about making multiple computers look like a single hosting space for objects.
And these things only fit into a service-oriented setting inside a service, never between them. The whole point of service-orientation is that services are autonomous and thus don’t trust each other. Transactions (including work-arounds like optimistic concurrency) are high-trust operations and thus can not exist between autonomous services – they can only exist inside them.
Personally I don’t think this rules out n-layer or n-tier applications, or n-layer and n-tier services. But I do think it reveals how the overall service-oriented may be fundamentally different from n-tier (or OO for that matter).
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