Availability zones allow for the automatic and uniform distribution of an application’s units across a region of the cloud where the units of the application have been deployed. Juju supports such zones on Google Compute Engine, VMware vSphere, Amazon’s EC2, OpenStack-based clouds, and MAAS .
To evenly distribute an application’s units across all available zones in a cloud region, Juju allocates a new unit to the zone that has the fewest number of units of that application already deployed.
See the Supported clouds page for more details on cloud regions and zones, and other cloud-specific settings.