Data Access Patterns

Sources: Martin Fowler — PoEAA (2002); Eric Evans — Domain-Driven Design (2003)

Pain Signals — You Are in This Branch When:

• db.query(...) or ORM calls appear inside service classes, domain objects, or route handlers
• Tests that cover business logic require a live database connection to run
• ORM model instances (e.g. SQLAlchemy models, ActiveRecord objects) are passed across layer boundaries into the domain
• "Which table do I query for this?" decisions appear inside business logic
• Changing the storage backend (e.g. Postgres → DynamoDB) would require touching domain code
• The same query is written in multiple places and diverges over time

Pattern Selection