Module Boundaries

Boundary Rules

Allowed: - module -> itself - module -> core - module -> shared

Forbidden: - module -> another module

Enforcement Intent

This rule exists to keep domains independently maintainable and future-extractable.

Consequences of Violation

• Hidden business coupling
• Circular design pressure
• Refactor blast radius grows
• Teams block each other on unrelated changes

Practical Example

Good:

from racerapi.modules.users.repo import UserRepo
from racerapi.core.exceptions import NotFoundError

Bad:

from racerapi.modules.health.service import HealthService

DO

• Duplicate tiny logic if it avoids domain coupling.
• Move generic, domain-neutral helpers to shared.
• Move cross-cutting policy to core.

DON'T

• Centralize domain behavior in shared for convenience.
• Read another module's tables/repo directly.

Common Mistakes

• Importing another module's schemas because they “already exist”.
• Reusing another module's repo query method.
• Building orchestration logic in API layer to stitch two modules.

When to Break the Rule

Only during planned strangler migrations with: - explicit temporary adapter, - defined end date, - tests proving no functional drift.