Horizontal scaling
Horizontal scaling (scale out) involves adding identical instances (servers, containers) to absorb growing load, rather than increasing the resources of a single server (vertical scaling). It requires stateless application architecture to distribute requests correctly.
In practice
A horizontally scalable architecture requires stateless applications: no session state stored locally (use Redis or a shared database), no uploaded files stored on local disk (use S3). A Next.js site deployed on Vercel or across multiple Docker containers behind a load balancer is horizontally scalable by default — adding an instance takes under a minute when facing a viral traffic spike.