Why Scale?
Scalability is the capability of a system, process, or network to grow and manage increased demand. As demand increases, can your system handle it? If it can, it's scalable. Any distributed system that can continuously evolve to support a growing amount of work is considered scalable. This is a good problem to have—you want your system to grow to the point where it needs to support more users. And when that happens, it needs to be scalable.
The Single-Server Problem
Think back to the original single-server setup. If you have a lot of clients sending requests to one server, that server will become overloaded. It won't be able to serve all the requests. Latency and response times will go up, causing a bad experience for users. To keep latency and response times low, you need to scale your server. There are two main ways to do that: vertically and horizontally.