Separation of Concerns

Separation of Concerns (SoC) is the principle that a system should be divided into distinct sections, each addressing a separate concern. It’s the foundation of maintainable software.

Why It Matters

• Change isolation: A change in one concern shouldn’t ripple across the system.
• Testability: Each concern can be tested independently.
• Comprehension: Developers can understand one concern without understanding everything.

In Practice

Horizontal Separation (Layers)

┌──────────────────────┐
│   Presentation       │  ← UI, HTTP handlers, CLI
├──────────────────────┤
│   Business Logic     │  ← Domain rules, workflows
├──────────────────────┤
│   Data Access        │  ← Database, external APIs
└──────────────────────┘

Vertical Separation (Bounded Contexts)

┌─────────┐  ┌─────────┐  ┌─────────┐
│ Orders  │  │ Billing │  │ Shipping│
│  ┌───┐  │  │  ┌───┐  │  │  ┌───┐  │
│  │UI │  │  │  │UI │  │  │  │UI │  │
│  ├───┤  │  │  ├───┤  │  │  ├───┤  │
│  │BL │  │  │  │BL │  │  │  │BL │  │
│  ├───┤  │  │  ├───┤  │  │  ├───┤  │
│  │DA │  │  │  │DA │  │  │  │DA │  │
│  └───┘  │  │  └───┘  │  │  └───┘  │
└─────────┘  └─────────┘  └─────────┘

Anti-Patterns

• God object: One class/module does everything.
• Shotgun surgery: A single change requires edits in many places.
• Leaky abstraction: Business logic bleeds into the data layer or vice versa.

Rules of Thumb

1. If a class has more than one reason to change, split it.
2. If a function does I/O and computation, separate them.
3. If a module imports from every other module, it’s a concern magnet — refactor.

Next: Layered Architecture