'Describes Amazon RDS instances.
**Required Permissions**: To use this action, an IAM user must
have a Show, Deploy, or Manage permissions level for the
stack, or an attached policy that explicitly grants
permissions. For more information on user permissions, see
`Managing User Permissions`_.
:type stack_id: string
:param stack_id: The stack ID that the instances are registered with.
The operation returns descriptions of all registered Amazon RDS
instances.
:type rds_db_instance_arns: list
:param rds_db_instance_arns: An array containing the ARNs of the
instances to be described.'

def describe_rds_db_instances(self, stack_id, rds_db_instance_arns=None):

    params = {'StackId': stack_id}
    if (rds_db_instance_arns is not None):
        params['RdsDbInstanceArns'] = rds_db_instance_arns
    return self.make_request(action='DescribeRdsDbInstances', body=json.dumps(params))
