Layers with dependency rules
Day 113 of 149
👉 Full deep-dive with code examples
The Onion Analogy
Think of an onion:
- Layers wrap around a core
- Inner layers don't know about outer layers
- You can peel off outer layers without affecting the inside
Clean Architecture organizes code like an onion!
The core business logic doesn't know about databases, web frameworks, or UI.
The Problem It Solves
Messy code has everything tangled together:
- Database logic mixed with business rules
- UI code calling databases directly
- Changing one thing breaks everything else
This makes code hard to:
- Test
- Change
- Understand
- Maintain
The Layers
From inside out:
1. Entities (Center):
- Core business objects
- Pure business rules
- No external dependencies
2. Use Cases:
- Application-specific business rules
- "User wants to place an order"
3. Interface Adapters:
- Convert data between layers
- Controllers, presenters, gateways
4. Frameworks & Drivers (Outer):
- Databases, web frameworks, UI
- The "details"
Why Layers Matter
The key rule: Dependencies point inward.
UI → Controller → Use Case → Entity
↓
Database Adapter → DB
Inner layers shouldn't know about outer layers:
- Business logic doesn't know you're using PostgreSQL
- Change database? Only change the outer layer
- Test business logic without the database
Benefits
- Testable → Test core logic without frameworks
- Flexible → Swap out database, UI, or framework
- Understandable → Clear separation of concerns
- Maintainable → Changes don't ripple everywhere
In One Sentence
Clean Architecture separates code into layers where inner core business logic is protected from outer details like databases and frameworks.
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