1.1.1 Horizontal Scaling
Scale-out: more replicas behind a balancer. Requires the app to be mostly stateless.
Why it matters
Vertical scaling hits a ceiling (one box, one failure domain). Horizontal scaling is how the web grew.
How it is used
Put session and files in shared stores. Make each instance disposable. Databases scale out later via read replicas and sharding — that is harder than scaling the app tier.