What types of scaling exist?

There are two main types of database scaling:

1. Vertical scaling (scale up), increasing a single server's power. For example: adding more RAM, a faster CPU, an SSD. Pro: simple to implement. Con: there's a physical ceiling, a server can't be strengthened indefinitely.
2. Horizontal scaling (scale out), adding new servers to spread the load. For example: data and requests get split across several machines (sharding, replication). Pro: higher overall performance and fault tolerance. Con: harder to configure and administer.