OpenStack was designed to be horizontally scalable; we have already seen how its services have been widely distributed in two concepts: cloud controllers and compute nodes.
The concept of cloud controllers aims to provide one or many kinds of central management and control over your OpenStack deployments. We can, for example, assume that all authentication and messaging transactions are being managed by the cloud controller by means of our magic hub: the message queue.
Considering a medium- or large-scale infrastructure, we will need, with no doubt, more than a single node. For an OpenStack cloud operator, controllers can be thought of as a service aggregator where the majority of running management services are needed for OpenStack to operate.
Let's see what a cloud controller cloud mainly handles:
It presents a main gateway for access to cloud management and services consumption
It provides the API services in order to bring different OpenStack components...