Kas saaksite selgitada Onion arhitektuuri põhimõtteid ja struktuuri tarkvarasüsteemide arendamisel?

Onion architecture is an approach to building software systems that focuses on the separation of responsibilities and the independence of business logic from external layers such as UI, databases, and infrastructure.

Main principles:

• The core of the architecture is the domain model and business rules.
• Around the domain, layers are arranged that depend only on the inner layer, not vice versa.
• External layers (e.g., infrastructure, UI) depend on internal layers through abstractions (interfaces).

Layer structure (from inside out):

1. Domain layer — entities, business rules.
2. Application layer — services implementing business logic, using domain objects.
3. Infrastructure layer — implementation of data access, external services.
4. User interface — interaction with the user.

This approach facilitates testing, maintenance, and extension of the system, as changes in external layers minimally affect the business logic.