Can you explain the principles and structure of Onion architecture in software development?

Onion architecture is an approach to building software systems aimed at separating responsibilities and making business logic independent of external layers such as UI, databases, and infrastructure.

Main principles:

• The core of the architecture is the domain model and business rules.
• Layers are arranged around the domain, depending 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, support, and system extension, as changes in external layers minimally affect business logic.