Clean / Onion / Hexagonal Architecture

Problem

Business rules become mixed with: HTTP SQL Azure Kafka UI Soon the business logic cannot be understood or tested without all of its infrastructure.

Solution — Isolate the Business Core

Onion places the domain at the center, application logic around it, and infrastructure—including persistence—outside; code dependencies point inward. Clean Architecture similarly points dependencies inward, separating entities (business rules), use cases (application actions), interface adapters, and outer frameworks. Hexagonal (Ports and Adapters) connects the core through interfaces: for example, an OrderRepository interface implemented by a SQL adapter. Both approaches can work together.

       HTTP
        ↓
     Adapter
        ↓
     Domain
        ↑
     Adapter
        ↑
     Database

Core idea: isolate core business logic from changing external technologies.