code: @Override
    public long getQueueSize() {
      Batch snapshot = openBatch;
      return executorQueue.size() * batchSize + (snapshot == null ? 0 : snapshot.size());
    }
docs: Gets the best estimate for items in the queue still awaiting processing.
  Since the value returned is a rough estimate, it can sometimes be more than
  the number of items actually in the queue but not less.
  
  @return the amount of elements still awaiting processing.