10A language you can prove

Evidence is only as good as the precision of what it proves. A requirement that says "the system shall ensure compatibility" cannot be verified; one that states exactly which product may be given to which recipient, under which conditions, can. The ubiquitous language is what gives requirements, tests and code that precision, and keeps them aligned.

One sentence, three artefacts

In a product built with DDD, a rule stated by a domain expert travels without translation. The same words appear in the backlog item, in the acceptance test and in the code. Consider a rule from the bedside administration context, written as an executable scenario:

Scenario: An incompatible unit is refused at the bedside
  Given a recipient whose blood group is O RhD negative
  And a red cell unit of group A RhD positive allocated to that recipient
  When the nurse scans the unit at the bedside
  Then administration is blocked
  And an "incompatibility detected" event is recorded

A haematologist can read and approve this scenario. A developer can automate it. An assessor can follow it from the requirement it verifies to the code it exercises. Because recipient, unit, allocated and administration have one meaning inside the context, there is no gap between what the expert asked for and what the test checks.

Traceability, which teams in regulated industries often maintain by hand in large matrices, becomes much lighter when names match. The link from requirement to code is no longer an entry in a spreadsheet; it is the same word in both places.

Rules the code cannot break

DDD's tactical patterns, often dismissed as boilerplate, earn their place in critical contexts. An aggregate is the unit that enforces a set of rules, its invariants, every time its state changes. If "a unit may not be administered without a valid final check" is an invariant of the administration aggregate, no screen, service or batch job can bypass it, because there is no way to change the state except through the aggregate.

Value objects make invalid values impossible to create. A blood group that can only be one of the defined groups, or a unit identifier that validates its own format and checksum, removes whole classes of defects before any test runs. Domain events, such as "unit issued", "unit administered" or "reaction reported", record what happened in business terms, with who and when. In a domain that requires traceability from donor to recipient, those events are both the integration mechanism between contexts and the audit trail.

Table 6 · DDD constructs read as assurance mechanisms.

Evidence produced sprint by sprint

This is where the combination with Scrum pays off. When the evidence is a by-product of how the code is written, the Definition of Done can require it at no great extra cost: scenarios approved, invariants tested, events documented, context map updated. The documentation file grows with each increment instead of being reconstructed before an audit, which is precisely the discipline Scrum in Name Only identified as the real challenge of regulated Scrum.