# Scalability

Scalability is the capability of a system, process, or a network to grow and manage increased demand. Any distributed system that can continuously evolve in order to support the growing amount of work is considered to be scalable.

A system may have to scale because of many reasons like increased data volume or increased amount of work, e.g., number of transactions.

**Horizontal vs. Vertical Scaling**:
Horizontal scaling means that you scale by adding more servers into your pool of resources whereas
Vertical scaling means that you scale by adding more power (CPU, RAM, Storage, etc.) to an existing server.

Good examples of horizontal scaling are Cassandra and MongoDB as they both provide an easy way to scale horizontally by adding more machines to meet growing needs.

Similarly, a good example of vertical scaling is MySQL as it allows for an easy way to scale vertically by switching from smaller to bigger machines. However, this process often involves downtime.