Scalability is the ability to add capacity to a system dynamically, without modifying its architecture. There are two types of scalability, as follows:

• Horizontal
• Vertical

Proactive scalability is when automatic means are in place to compensate the system capacity via monitoring (before the event occurs).

Reactive scalability is when the event has occurred and manual intervention is put in place to compensate the capacity (when the alarm threshold has been reached).