You got it. The only contract you have between tiers is the interface (methods, parameters and return values). As long as the interface doesn't change, you can switch tiers without affecting the others.

>Yeah, I'm starting to get my head around this now.
>
>So providing method/procedure names etc. are the same, it doesn't really matter what it's written in, as long as it retrieves/returns what is it expected of the data/front tier?