@Override
  public ValidTxnWriteIdList getValidWriteIds(List<String> tableList,
                                              String validTxnList) throws LockException {
    assert isTxnOpen();
    if (!StringUtils.isEmpty(validTxnList)) {
      try {
        return TxnCommonUtils.createValidTxnWriteIdList(txnId, getMS().getValidWriteIds(tableList, validTxnList));
      } catch (TException e) {
        throw new LockException(ErrorMsg.METASTORE_COMMUNICATION_FAILED.getMsg(), e);
      }
    }
    return null;
  }
Start the heartbeater threadpool and return the task.
@param initialDelay time to delay before first execution, in milliseconds
@return heartbeater
ql/src/java/org/apache/hadoop/hive/ql/lockmgr/DbTxnManager.java