003 – Schema per Module

Status	Accepted
Concerns	Database, Data Isolation, Module Boundaries

Context

All three backend modules share a single PostgreSQL instance (one deployment unit). Without a boundary strategy, modules could freely read each other's tables, creating hidden coupling that would make future extraction into independent services very difficult.

Decision

Each module owns exactly one dedicated PostgreSQL schema:

Module	Schema
Core	core
Operation	operation
Registration	registration

Rules:

• A module's repositories only access their own schema.
• No module may query another module's schema directly, even via raw SQL.
• Cross-module data needs must go through the module's internal API.
• Each module manages its own schema migrations independently via Flyway.

Consequences

• Schema isolation enforces the same boundary at the data layer as at the API layer.
• If a module is extracted into an independent service, its schema can be migrated to a dedicated database without changes to the data model.
• Join queries across module boundaries are not possible — any aggregation must happen at the application layer.