Architecture & practice·Design, structure and tradeoffs
you swapped the storage backend and did not have to open a single pricing rule.
Orthogonality
Two parts are orthogonal when changing one has no effect on the other. It is the property that makes a system possible to reason about piecemeal, because the number of interactions you have to consider stops growing with the number of components. In a design discussion it is the strongest argument for a boundary: not that it is tidy, but that it makes a whole class of change local.