availability
'Availability type of an Instance. Defaults to REGIONAL for both primary and read instances. Note that primary and read instances can have different availability types. Only READ_POOL instance supports ZONAL type. Users can't specify the zone for READ_POOL instance. Zone is automatically chosen from the list of zones in the region specified. Read pool of size 1 can only have zonal availability. Read pools with node count of 2 or more can have regional availability (nodes are present in 2 or more zones in a region).' Possible values are: AVAILABILITY_TYPE_UNSPECIFIED, ZONAL, REGIONAL.