B.2 Derive; Don't Copy

Problem. An authoritative source loses authority when consumers copy its values into independently maintained snapshots. The model may remain nominally authoritative while engineering decisions operate on stale copies.

Move. Consume authoritative representations by query or derivation rather than creating a second editable copy.

Figure B.2-1 sets the snapshot against the query.

C.2 Derive, don't copy: a snapshot goes stale while a live query stays current Two ways a consumer reads an authoritative model. In the bad path, the consumer copies values into a snapshot and checks against the snapshot; when the model changes, the snapshot does not, so the check runs against a stale answer. In the good path, the consumer queries the model directly; when the model changes, the next check sees the change because the consumer is coupled to the model's interface, not to yesterday's answer. MODEL copy SNAPSHOT CHECK the model changes… … the snapshot does not — the check trusts a stale answer GOOD · query at the point of use MODEL query — no second editable truth CHECK the model changes… … the next check sees it — coupled to the interface, not yesterday's answer Consume authority by query or derivation; a snapshot is a new truth free to drift.
Figure B.2-1. Query, don't snapshot. BAD — the model is copied into a snapshot that a check reads; the model changes, the snapshot does not. GOOD — the check queries the model directly, so the next check sees the change.

Example — Live model consumption. Model-aware lints, tests, and agent briefs query the current representation instead of embedding copied values. When the modeled architecture changes, consumers see the new state through the model interface; there are no stale copies to find and update.

Example — Generated artifacts. Consumers that should not query at run time can instead be generated from the authoritative model at build time. The downstream artifact is still derived from the model rather than maintained as a second source of truth.

Related mechanisms: Meta-model consumption discipline · Model query surface · Model-driven codegen · Doc-hygiene provenance.

© James C. Davis, 2026–present