Vertical vs horizontal scaling and stateless services

There are two ways to handle more traffic. Vertical scaling, or scale up, means a bigger machine with more CPU, more memory or faster disks. It is simple and needs no code changes.

But one machine can only get so big, and if it dies, everything stops: it is a single point of failure. Horizontal scaling, or scale out, means adding more machines behind a load balancer, which shares requests between them. There is no hard ceiling, and the system survives one machine dying.

It only works if any server can handle any request. That is why we build stateless services: a server keeps nothing in its own memory between requests. Sessions, uploads and caches move out of the app into a shared store such as Redis, a database or object storage.