Horizontal scaling
Also called: Scaling out
Handling more load by adding more machines that do the same job, instead of buying a bigger one.
Definition
Scaling out (horizontally) adds servers behind a load balancer; scaling up (vertically) replaces a server with a bigger one. Horizontal scaling has no practical upper limit and survives single-server failures, but only works if servers are stateless: sessions, carts and files must live in shared stores, so any server can handle any request.
In practice
Going from 2 to 10 identical API servers during a sale, with carts stored in Redis instead of server memory.
Learn it in context
Lessons that use this term.