Onion Architecture

Onion Architecture models concentric layers around the domain core so infrastructure depends on application and domain, never the reverse. Domain services and aggregates sit near the center; application services coordinate use cases; infrastructure implements repositories and messaging at the outside.

On interviews: compare Onion with Hexagonal and Clean Architecture—they share the dependency rule with different vocabulary. Discuss where application services end and domain services begin.

Common pitfalls: onion diagrams with hollow centers; domain services that only wrap repositories; infrastructure leaking into application via shared ORM models.

The trade-off is flexibility versus complexity—know when the simpler path is enough.

Checklist:

• Domain model expresses business language.
• Application layer coordinates transactions.
• Infrastructure references inward interfaces only.
• Tests target domain without database when possible.