Vertical scaling is basically makes the nodes stronger.

▪ Make the server stronger with adding more hardware. Adding more

resources to a single node.

▪ Make optimization the hardware that will allow you to handle more

requests.

▪ Vertical scaling keeps your existing infrastructure but adding more

computing power.

▪ Your existing code doesn't need to change.

▪ Adding additional CPU, RAM, and DISK to cope with an increasing

workload.

▪ By scaling up, you increase the capacity of a single machine. And it has

limits. That is named Scalability Limits.

▪ Because even the hardware has maximum capacity limitations.

▪ For handling millions of request, we need horizontal scaling or scaling out.

https://www.section.io/blog/scaling-horizontally-vs-vertically/