def apply_pending_maintenance_action(ResourceIdentifier=None, ApplyAction=
    None, OptInType=None):
    """
    Applies a pending maintenance action to a resource (for example, to a DB instance).
    See also: AWS API Documentation
    
    Examples
    This example immediately applies a pending system update to a DB instance.
    Expected Output:
    
    :example: response = client.apply_pending_maintenance_action(
        ResourceIdentifier='string',
        ApplyAction='string',
        OptInType='string'
    )
    
    
    :type ResourceIdentifier: string
    :param ResourceIdentifier: [REQUIRED]
            The RDS Amazon Resource Name (ARN) of the resource that the pending maintenance action applies to. For information about creating an ARN, see Constructing an RDS Amazon Resource Name (ARN) .
            

    :type ApplyAction: string
    :param ApplyAction: [REQUIRED]
            The pending maintenance action to apply to this resource.
            Valid values: system-update , db-upgrade
            

    :type OptInType: string
    :param OptInType: [REQUIRED]
            A value that specifies the type of opt-in request, or undoes an opt-in request. An opt-in request of type immediate cannot be undone.
            Valid values:
            immediate - Apply the maintenance action immediately.
            next-maintenance - Apply the maintenance action during the next maintenance window for the resource.
            undo-opt-in - Cancel any existing next-maintenance opt-in requests.
            

    :rtype: dict
    :return: {
        'ResourcePendingMaintenanceActions': {
            'ResourceIdentifier': 'string',
            'PendingMaintenanceActionDetails': [
                {
                    'Action': 'string',
                    'AutoAppliedAfterDate': datetime(2015, 1, 1),
                    'ForcedApplyDate': datetime(2015, 1, 1),
                    'OptInStatus': 'string',
                    'CurrentApplyDate': datetime(2015, 1, 1),
                    'Description': 'string'
                },
            ]
        }
    }
    
    
    """
    pass
