code: public Transaction startLong() {
      txMetricsCollector.rate("start.long");
      Stopwatch timer = new Stopwatch().start();
      long expiration = getTxExpiration(defaultLongTimeout);
      Transaction tx = startTx(expiration, TransactionType.LONG);
      txMetricsCollector.histogram("start.long.latency", (int) timer.elapsedMillis());
      return tx;
    }
docs: Start a long transaction. Long transactions and do not participate in conflict detection. Also, aborting a long
  transaction moves it to the invalid list because we assume that its writes cannot be rolled back.