code: public void offerBackup(Data data, long itemId) {
          QueueItem item = new QueueItem(this, itemId, null);
          if (!store.isEnabled() || store.getMemoryLimit() > getItemQueue().size()) {
              item.setData(data);
          }
          getBackupMap().put(itemId, item);
      }
docs: Offers the item to the backup map. If the memory limit
  has been achieved the item data will not be kept in-memory.
  Executed on the backup replica
  
  @param data   the item data
  @param itemId the item ID as determined by the primary replica