What is the difference between top-down and bottom-up design?

Question

Accepted Answer

**Top-down** — start with high-level, break down. **Bottom-up** — start with components, build up.

**Top-Down:**
- Start with system overview.
- Decompose into subsystems.
- Continue to components.
- Good for new systems.

```
Top-Down:
E-Commerce System
  ├── User Module
  │   ├── Authentication
  │   └── Profile
  ├── Order Module
  └── Payment Module
```

**Bottom-Up:**
- Start with existing components.
- Combine into subsystems.
- Build up to system.
- Good for integration.

```
Bottom-Up:
[Auth Lib] + [Payment Lib] + [Order Lib]
    → Combined into E-Commerce System
```

**Key differences:**
- **Top-down** — high-level first.
- **Bottom-up** — components first.
- **Top-down** — new systems.
- **Bottom-up** — integration.

Systems Analyst: What is the difference between top-down and bottom-up design?