[lliamander]

Good point. The solution is obviously going to be context-specific. I suspect in our hypothetical order scenario that the order fulfillment system could get away with not validating the product being ordered.

Of course it would still need a way of capturing the identity of that product. I would guess that the best way to handle that would be to pass a URN or URI for the product. That would still decouple it somewhat from the product catalog system.

In general for these kinds of integrations, you're probably better off avoiding passing around system specific IDs. Pass data instead of IDs (pass the geocordinates or standardized address for a customers delivery address rather than the customer ID). If you have to pass around IDs, then URIs allow for greater decoupling.