Ken-

First, there's always a difference between talking abstract vs. actual implementation. So, part of design is deciding what you have to work with.

Second, that RI is a business rule doesn't mean it can't *live* in a database

But, by keeping your RI in your database (coupling it?), I think you tie yourself to using one backend.

The whole point to having a component approach, AFAICT, is to be able to call the rule from wherever.

Keep in mind, this is me talking and I'm also struggling with these issues.