code: public void pollBackup(long itemId) {
          QueueItem item = getBackupMap().remove(itemId);
          if (item != null) {
              // for stats
              age(item, Clock.currentTimeMillis());
          }
      }
docs: Polls an item on the backup replica. The item ID is predetermined
  when executing the poll operation on the partition owner.
  Executed on the backup replica
  
  @param itemId the item ID as determined by the primary replica