What is the difference between vertical and horizontal scaling?

Vertical scaling (scaling up) — increasing resources of a single server (CPU, RAM, disk space). It is used when the current server becomes a bottleneck.

Horizontal scaling (scaling out) — adding new servers to distribute the load. It is used to improve system availability and performance through parallel request processing.

Characteristic	Vertical scaling	Horizontal scaling
Approach	Enhancing a single server	Adding new servers
Implementation complexity	Simpler, often requires a reboot	More complex, requires load and data distribution
Scaling limit	Limited by hardware maximums	Theoretically unlimited
Impact on availability	Requires downtime for upgrade	Increases availability (fault tolerance)
Cost	High price for top-tier hardware	Linear growth with server costs
Typical application	Increasing database and monolithic application performance	Web servers, microservices, distributed systems