def describe_rds_db_instances(StackId=None, RdsDbInstanceArns=None):
    """
    Describes Amazon RDS instances.
    This call accepts only one resource-identifying parameter.
    See also: AWS API Documentation
    
    
    :example: response = client.describe_rds_db_instances(
        StackId='string',
        RdsDbInstanceArns=[
            'string',
        ]
    )
    
    
    :type StackId: string
    :param StackId: [REQUIRED]
            The stack ID that the instances are registered with. The operation returns descriptions of all registered Amazon RDS instances.
            

    :type RdsDbInstanceArns: list
    :param RdsDbInstanceArns: An array containing the ARNs of the instances to be described.
            (string) --
            

    :rtype: dict
    :return: {
        'RdsDbInstances': [
            {
                'RdsDbInstanceArn': 'string',
                'DbInstanceIdentifier': 'string',
                'DbUser': 'string',
                'DbPassword': 'string',
                'Region': 'string',
                'Address': 'string',
                'Engine': 'string',
                'StackId': 'string',
                'MissingOnRds': True|False
            },
        ]
    }
    
    
    """
    pass
