OpenStack Partitioning - Regions, Availability Zones and Host Aggregates
An OpenStack Cloud can be divided into three main hierarchical zones - Regions, Availability Zones and Host Aggregates.
A Region is full OpenStack deployment, including its own API endpoints, networks and compute resources, excluding the Keystone and Horizon. Each Region shares a single set of Keystone and Horizon services.
Host Aggregates are logical groups of compute nodes. Each aggregate consists of compute nodes and the relating metadata. Only administrators are able to view or create host aggregates. An aggregates metadata is commonly used to provide information for use with the Compute scheduler (for example, limiting specific flavors or images to a subset of hosts). Metadata specified in a host aggregate will limit the use of that host to any instance that has the same metadata specified in its flavor.
An Availability Zone (AZ) is a logical regional grouping of compute nodes. Different AZs can be configured across multiple locations , providing high availability. Unlike Host Aggregates, Availability Zones are exposed to end users who can select a particular availability zone for VM placement while the instance is launched.
Figure 1 - OpenStack Partitioning - source : http://nubla.do/segregation
 "OpenStack - Regions and Availability Zones - GURU Advisor." http://www.guruadvisor.net/en/cloud/121-openstack-regions-and-availability-zones. Accessed 26 Aug. 2017.
 "3.4. Manage Host Aggregates - Red Hat Customer Portal." https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux_OpenStack_Platform/6/html/Administration_Guide/section-host-aggregates.html. Accessed 26 Aug. 2017.
 "Why use Openstack Cloud ?: OpenStack Cloud Partitioning ...." 25 Jun. 2014, http://whyuseopenstack.blogspot.com/2014/06/openstack-cloud-partitioningavailabilit.html. Accessed 26 Aug. 2017.