public int getPipeEventCount() {
    return inputPendingQueue.size()
        + prefetchingQueue.stream()
            .map(SubscriptionEvent::getPipeEventCount)
            .reduce(Integer::sum)
            .orElse(0)
        + inFlightEvents.values().stream()
            .map(SubscriptionEvent::getPipeEventCount)
            .reduce(Integer::sum)
            .orElse(0);
  }
@return {@code true} if nack successfully
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/subscription/broker/SubscriptionPrefetchingQueue.java