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**Description**

From Cloud Bigtable [documentation](https://cloud.google.com/bigtable/docs/replication-overview#how-it-works)
> Cloud Bigtable supports up to 4 replicated clusters located in Google Cloud zones where Cloud Bigtable is available.
Currently Bigtable hook and operator only support creating one replicated cluster. It would be better to add support for creating multiple replicated clusters

**Use case / motivation**

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Supporting multiple replicated clusters will be handy. It will be helpful to increase the availability and durability of the data by copying it across multiple regions or multiple zones within the same region.


airflow/providers/google/cloud/operators/bigtable.py
"""
    Common class for Cloud Bigtable operators for validating required fields.
    """
    REQUIRED_ATTRIBUTES = []  
    def _validate_inputs(self):
        for attr_name in self.REQUIRED_ATTRIBUTES:
            if not getattr(self, attr_name):
                raise AirflowException('Empty parameter: {}'.format(attr_name))
class BigtableCreateInstanceOperator(BaseOperator, BigtableValidationMixin):
    """
    Creates a new Cloud Bigtable instance.
    If the Cloud Bigtable instance with the given ID exists, the operator does not
    compare its configuration
    and immediately succeeds. No changes are made to the existing instance.
    For more details about instance creation have a look at the reference:
    https://googleapis.github.io/google-cloud-python/latest/bigtable/instance.html#google.cloud.bigtable.instance.Instance.create
    .. seealso::
        For more information on how to use this operator, take a look at the guide:
        :ref:`howto/operator:BigtableCreateInstanceOperator`
    :type instance_id: str
    :param instance_id: The ID of the Cloud Bigtable instance to create.
    :type main_cluster_id: str
    :param main_cluster_id: The ID for main cluster for the new instance.
    :type main_cluster_zone: str
    :param main_cluster_zone: The zone for main cluster