id: ADR-0026 title: Drop uom and arrow-flight; pint validates what the adapter emits and never defines a quantity status: accepted date: 2026-09-13 deciders: [paul-heyse] level: decision principles: [DM-02, DM-42, DM-58, DM-43] blueprint: [§3.1, §3.3, §21.2] review: docs/design_review/reviews/design_review_arrow-native-idaes-core-blueprint-rev2_2026-09-13.md#7-principle-findings evidence: Interface-checked supersedes: [] superseded-by: null revisit: A kernel genuinely needs compile-time unit typing, or a remote Arrow transport becomes a requirement verification: rust / deny (deny.toml bans uom and arrow-flight); python / lint banned-api rule on assert_units_consistent; tests/governance/tests/banned_patterns.rs

ADR-0026: Drop uom and arrow-flight; pint validates what the adapter emits and never defines a quantity

Context

Blueprint §3.3’s “explicitly not added” list drops uom and arrow-flight, and the second review’s R2-4 found §3.3 and §21.2 contradicting each other about whether a Python units library is an authority.

Scope

Binds the units authority and removes two dependencies with no consumer. The quantity-type contract itself is ADR-0008.

Drivers

One authority per fact (DM-02): the registry defines quantities and the unit set; uom’s stock SI system cannot express the currency dimension costing needs; arrow-flight has no consumer, and a dependency with no consumer is a supply-chain surface for nothing.

Options

Compile-time unit typing in kernels with uom — rejected: KernelSpec signatures are checked by P10’s unit inference and by the per-kernel parity suites, and the currency dimension does not fit. Keep arrow-flight for a future service — rejected: add it when a service exists.

Outcome

uom and arrow-flight are banned in deny.toml. pint is present only because Pyomo’s units machinery is pint: it validates what the adapter emits and never defines a quantity, and no relation is ever derived from a pint object.

Consequences

The adapter calls pyomo.util.check_units.identify_inconsistent_units — not a bare assert_units_consistent — so a disagreement names the offending components in a compile.math finding; if the two disagree, the adapter has a bug.

Compensating controls

A ruff banned-api rule rejects assert_units_consistent in the Python package; deny.toml carries both bans with reasons and review dates.

Confirmation

python / lint and rust / deny are required checks; the unit-inference tests in tests/conformance are the positive side.

Pros and cons

Dropping uom means unit errors are caught by inference rather than by the type system; the currency dimension makes the type-system route unavailable anyway.

More information

Blueprint §3.1, §3.3 (“explicitly not added”), §21.2 (adapter algorithm step 1); review findings R2-4 and R2-5.

Status history

• 2026-09-13 — accepted with the repository-seeding pull request (backfilled from blueprint revision 3).