Regions, cells, and availability zones
As we mentioned before, OpenStack is designed to be scalable, but not infinitely scalable. There are three different techniques architects can use to segregate an OpenStack cloud: regions, cells, and availability zones. In this section, we'll walk through how each of these concepts maps to hypervisor topologies.
Regions
From an end user's perspective, OpenStack regions are equivalent to regions in Amazon Web Services. Regions live in separate data centers and are often named after their geographical location. If your organization has a data center in Phoenix and one in Raleigh you'll have at least a PHX and a RDU region. Users who want to geographically disperse their workloads will place some of them in PHX and some of them in RDU. Regions have separate API endpoints, and although the Horizon UI has some support for multiple regions, they are essentially entirely separate deployments.
From an architectural standpoint, there are two main design choices...
