Availability Zones (AZs) are a special case of host aggregates. Here, host aggregates are used by Nova to make scheduling decisions; they are generally only visible to operators and administrators of the OpenStack Cloud. AZs are the end-user visible component. An AZ can be configured, like host aggregates, to represent features of the hardware.
AZs are typically used to define failure domains, such as a cabinet or data center, or even geography. When configuring AZs, it is important to note that a host can only be a member of one AZ at a time.
Note
As AZs are a special use-case of host aggregates, a lot of operations, such as adding and removing hosts are the same. Thus, they are not repeated here.
Creating an AZ is a two-step process. First, we create an aggregate with the --zone parameter, or add it to an existing aggregate. Second, a host needs to be added to the aggregate before instances can be scheduled into the zone.