You manage how the system scales by adding or removing instances to the system and also by specifying which services run on those instances.
Instances
An instance is a server or virtual machine on which the software is running. A system can have either a single instance or multiple instances. Multi-instance systems have a minimum of four instances.
A system with multiple instances maintains higher availability in the event of instance failures. Additionally, a system with more instances can run tasks concurrently and can typically process tasks faster than a system with fewer or only one instance.
A multi-instance system has two types of instances: master instances, which run an essential set of services, and non-master instances, which are called workers.
Services
Each instance runs a configurable set of services, each of which performs a specific function. For example, the Metadata Gateway service stores metadata persistently.
In a single-instance system, that instance runs all services. In a multi-instance system, services can be distributed across all instances.