# Boundary Rules

Hard constraints for every implementation phase. Violations block completion.

Canonical ownership: [module-boundaries.md](../architecture/module-boundaries.md). Process mandate: [ADR-018](../architecture/adr/ADR-018.md).

## Service & Module Boundaries

1. **Never implement responsibilities owned by another service.**
2. **Never duplicate business logic** that already lives in an owning service — consume it.
3. **Never access another service’s database** (no cross-DB queries, FKs, or shared schemas).
4. **Integrate only through** versioned HTTP APIs, domain/integration Events, and Provider interfaces.
5. **Never import** another service’s models, repositories, or private modules.

## Phase Boundaries

1. **Never move functionality from future phases into the current phase.**
2. **Never silently expand Scope** into adjacent modules “while we’re here.”
3. **Out of Scope is binding** — document exclusions and respect them.
4. High-level roadmap text authorizes **deriving production engineering** for the current phase ([definition-of-done.md](definition-of-done.md), [enterprise-phase-discovery.md](enterprise-phase-discovery.md)), **not** pulling next-phase product features forward.

## Data & Logic Boundaries

| Allowed | Forbidden |
| --- | --- |
| External ID / ref fields (`*_ref`) | Cross-service foreign keys |
| HTTP clients to published APIs | Reading foreign ORM models |
| Outbox → event bus → consumers | Dual-writing the same journal/aggregate in two services |
| Provider adapters in the **owning** service | Implementing SMS/payment/etc. inside a non-owning vertical |
| Shared-lib envelopes/exceptions/JWT helpers | Shared-lib business workflows |

## Accounting & Platform Specials

1. Journal entries only via Accounting Posting Engine ([ADR-010](../architecture/adr/ADR-010.md)).
2. Messaging providers only inside Communication ([ADR-012](../architecture/adr/ADR-012.md)).
3. Product AI remains optional and independent ([ai-architecture.md](../architecture/ai-architecture.md)).

## Frontend / Backend

1. UI only under `frontend/` ([ADR-002](../architecture/adr/ADR-002.md)).
2. Backend never embeds UI rules as the source of truth.
3. Frontend never imports backend packages.

## Enforcement

- Architecture Validation stage ([development-loop.md](development-loop.md))
- Architecture / Integration gates ([quality-gates.md](quality-gates.md))
- Architecture tests in the service suite ([testing-template.md](testing-template.md))

## Related Documents

- [Module Boundaries](../architecture/module-boundaries.md)
- [Enterprise Phase Discovery](enterprise-phase-discovery.md)
- [Master Prompt](master-prompt.md)
- [Service Template](service-template.md)
- [Module Template](module-template.md)