I don't understand how you would have a persistence layer or otherwise that can't be refactored without changing external code that depends on that layer. Batc

I don't understand how you would have a persistence layer or otherwise that can't be refactored without changing external code that depends on that layer. Batch is hard of course. But it's conceivable to have a crappy implementation (just looping single statements) and then later refactor it to do it database side, or perhaps in parallel (looping single statements simultanously). All the while without modifying the dependent code. No?