What is the difference between horizontal and vertical scaling in AWS?

Vertical Scaling (Scale Up): Increase the size of an existing instance (e.g., t3.medium → c5.4xlarge). Simple but has a ceiling (there’s a maximum instance size). Requires downtime to resize EC2.

Horizontal Scaling (Scale Out): Add more instances behind a load balancer. No theoretical ceiling. Enables high availability and fault tolerance because traffic is spread across multiple instances in multiple AZs.

AWS Auto Scaling Groups with Application Load Balancers enable fully automated horizontal scaling based on metrics like CPU or custom CloudWatch metrics.