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

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Scaling is how a system grows to handle more load. Two strategies — vertical (scale up) and horizontal (scale out) . Vertical scaling — make a single resource bigger. In AWS: change an EC2 instance type (e.g., m5.large → m5.4xlarge ) or RDS class. Simple, but requires downtime (stop/start) and hits a ceiling at the largest instance. Horizontal scaling — add more identical resources. In AWS: Auto Scaling Groups add EC2 behind an ALB, RDS Read Replicas, DynamoDB partitions. No ceiling, no downtime to scale, but requires stateless app design or shared state in caches/DBs. Cloud preference: horizontal scaling is the AWS default — it's elastic, resilient (multiple instances survive AZ failure), and cheaper at scale with Spot. Cloud-native = horizontal. Mention Auto Scaling Groups + ALB and stateless apps; that's the senior-engineer phrasing.