Act 5, scene 21: Drill-down: business rules and invariants

Scene 21: business rules and invariants

Then the business rules, which a brownfield programme reaches later, if at all. The language expresses each customer's rules in the domain's own terms, over the invariants: which entities exist, how they relate, and which workflows govern them. A customer's variation becomes that customer's own business rules, checked against the invariants, and the shared code stays as it is. The existing APIs may not offer the granularity the language needs, but they already cover a large part of the domain. Deciding what becomes an invariant is architectural judgement.