What is the difference between horizontal and vertical scaling?

Question

Accepted Answer

**Horizontal** — add more machines. **Vertical** — add more power to existing machine.

**Horizontal Scaling (Scale Out):**
- Add more servers.
- Load balancer distributes traffic.
- Linear cost increase.
- Better fault tolerance.

```
Horizontal:
  [Server 1]
  [Server 2]  ← Load Balancer
  [Server 3]
(Multiple servers)
```

**Vertical Scaling (Scale Up):**
- Upgrade existing server.
- Add CPU, RAM, storage.
- Limited by hardware.
- Single point of failure.

```
Vertical:
  [Server: 4 CPU → 8 CPU → 16 CPU]
(More powerful single server)
```

**Key differences:**
- **Horizontal** — more servers.
- **Vertical** — more power.
- **Horizontal** — better scalability.
- **Vertical** — simpler initially.

Systems Analyst: What is the difference between horizontal and vertical scaling?