What type of scaling involves adding servers to increase capacity?

Horizontal scaling refers to the process of adding more servers or instances to a resource pool in order to handle increased loads or improve performance. This approach is often used in cloud environments to enhance capacity and performance without needing to upgrade or replace existing servers. By distributing workloads across multiple machines, horizontal scaling not only boosts capacity but also provides benefits such as redundancy and higher availability.

In contrast, vertical scaling involves upgrading the existing server's resources—like CPU, RAM, or storage—rather than adding more servers. Elasticity is the capability of a system to dynamically allocate resources as demand changes, allowing for scaling both up and down, but it does not specifically reference the act of adding servers. Autoscaling is an automated process that adjusts the number of active servers based on current demand, but it can include both horizontal and vertical scaling strategies. Therefore, horizontal scaling is the most precise term for the method of adding servers to increase capacity.