How to use the persistence.get_backup_plan function in Persistence

def _process_plan(self, plan, plan_backups):
        total_dont_expire = 0
        total_expired = 0
        logger.info("==== Processing plan '%s' .... " % plan.id)
        # Ensure we have the latest revision of the backup plan
        plan = persistence.get_backup_plan(plan.id) or plan
        try:
            expirable_backups, non_expirable_backups = self.find_plan_expirable_backups(plan, plan_backups)
            if non_expirable_backups:
                mark_plan_backups_not_expirable(plan, non_expirable_backups)
                total_dont_expire += len(non_expirable_backups)

            total_expired += self.expire_plan_dues(plan, expirable_backups)
        except Exception, e:
            logger.exception("BackupExpirationManager Error while"
                             " processing plan '%s'" % plan.id)
            subject = "BackupExpirationManager Error"
            message = ("BackupExpirationManager Error while processing"
                       " plan '%s'\n\nStack Trace:\n%s" %
                       (plan.id, traceback.format_exc()))
            get_mbs().notifications.send_error_notification(subject, message)

def is_plan_backups_expirable(self, plan):
        # We only allow expiring backups that has a whose plans still exist
        #  and has a retention policy
        return persistence.get_backup_plan(plan.id) is not None

def validate_recurring_backup_expiration(self, backup):
        logger.info("Validating if recurring backup '%s' should be "
                    "expired now" % backup.id)
        # Ensure we have the latest revision of the backup plan when possible
        plan = persistence.get_backup_plan(backup.plan.id) or backup.plan

        rp = plan.retention_policy

        if not rp:
            raise BackupExpirationError(
                "Bad attempt to expire backup '%s'. "
                "Backup plan does not have a retention policy" % backup.id)
        occurrences_to_retain = \
            rp.get_plan_occurrences_to_retain_as_of(plan, date_now())

        if backup.plan_occurrence in occurrences_to_retain:
            raise BackupExpirationError(
                "Bad attempt to expire backup '%s'. Backup must not be"
                " expired now." % backup.id)
        else:
            logger.info("Backup '%s' good be expired now" %