code: public void txnPollBackupReserve(long itemId, String transactionId) {
          QueueItem item = getBackupMap().remove(itemId);
          if (item != null) {
              txMap.put(itemId, new TxQueueItem(item).setPollOperation(true).setTransactionId(transactionId));
              return;
          }
          if (txMap.remove(itemId) == null) {
              logger.warning("Poll backup reserve failed, itemId: " + itemId + " is not found");
          }
      }
docs: Makes a reservation for a poll operation. Should be executed as
  a part of the prepare phase for a transactional queue poll
  on the partition backup replica.
  The ID of the item being polled is determined by the partition
  owner.
  
  @param itemId        the ID of the reserved item to be polled
  @param transactionId the transaction ID
  @see #txnPollReserve(long, String)
  @see com.hazelcast.collection.impl.txnqueue.operations.TxnReservePollOperation