Mule developers build integration solutions that must cater to high volumes of real-time and batch data. In some cases, the size of payloads can be larger than memory. In others, API solutions may have to handle high rates of API calls per second.
Horizontal scalability, whereby Mule instances are deployed across multiple virtual and physical machines, is costly. Thus, a single Mule instance must utilize to the maximum underlying processing and storage resources as it handles traffic. Its ability to handle more traffic with more underlying resources is what we refer to as vertical scalability.