Class InstanceType (0.5.0) 

InstanceType(value)

Type of an Instance

    - 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).
SECONDARY (3):
    SECONDARY instances support read operations
    only. SECONDARY instance is a cross-region read
    replica

Enums
Name Description
INSTANCE_TYPE_UNSPECIFIED The type of the instance is unknown.
PRIMARY PRIMARY instances support read and write operations.
READ_POOL READ POOL instances support read operations only. Each read pool instance consists of one or more homogeneous nodes.