Explain Horizontal vs Vertical Scaling with Examples.

Horizontal scaling (scale-out) adds more servers to distribute the load, while vertical scaling (scale-up) increases a single machine’s capacity (CPU, RAM, storage) to handle more traffic.

When to Use

Use horizontal scaling when you need fault tolerance, high availability, or elastic growth—like scaling Netflix’s streaming servers or Amazon’s EC2 clusters. Use vertical scaling for smaller apps, monoliths, or databases where simplicity matters and downtime is manageable.

Example

A startup may upgrade its single database server (vertical scaling) initially, but as traffic grows, it distributes data across multiple servers (horizontal scaling).

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Why Is It Important

Scaling choices affect performance, cost, and reliability. Horizontal scaling offers near-infinite growth, while vertical scaling is quick and simple but limited by hardware.

Interview Tips

Clarify “scale-out vs scale-up” and discuss trade-offs. Use real examples (e.g., AWS Auto Scaling vs upgrading EC2 instance). Showing you understand when to use each approach stands out in interviews.

Trade-offs

• Horizontal scaling: +Elastic, fault-tolerant; –Complex (load balancing, consistency).
• Vertical scaling: +Simple; –Limited growth, potential downtime.

Pitfalls

• Assuming vertical scaling can scale indefinitely.
• Ignoring data distribution and consistency issues in horizontal scaling.