由於Hadoop 3.0.0 快接近正式釋出的階段，所以最近投票活動頻繁，參與投票也是社群活動的日常。
作者是個很懶的人，所以使用了Docker做了一個image的範本，只要把版本下載點替換後，rebuild image就可以幫我把Hadoop cluster建置起來，我可以利用Docker在建置的空擋做其他事情。
就在Hadoop 3.0.0RC0的Docker image建置完畢要啟動叢集時，發生了下列的錯誤訊息：
hadoop_master | 2017-11-16 02:56:15,560 INFO namenode.NameNode: SHUTDOWN_MSG:
hadoop_master | /************************************************************
hadoop_master | SHUTDOWN_MSG: Shutting down NameNode at hadoop_master/172.19.0.100
hadoop_master | ************************************************************/
hadoop_master | Starting namenodes on [hadoop_master]
hadoop_master | ERROR: Attempting to operate on hdfs namenode as root
hadoop_master | ERROR: but there is no HDFS_NAMENODE_USER defined. Aborting operation.
hadoop_master | Starting datanodes
hadoop_master | ERROR: Attempting to operate on hdfs datanode as root
hadoop_master | ERROR: but there is no HDFS_DATANODE_USER defined. Aborting operation.
hadoop_master | Starting secondary namenodes [hadoop_master]
hadoop_master | ERROR: Attempting to operate on hdfs secondarynamenode as root
hadoop_master | ERROR: but there is no HDFS_SECONDARYNAMENODE_USER defined. Aborting operation.
hadoop_master | 2017-11-16 02:56:19,817 WARN util.NativeCodeLoader: Unable to load native-hadoop library for your platform... using builtin-java classes where applicable
hadoop_master | Starting resourcemanager
hadoop_master | ERROR: Attempting to operate on yarn resourcemanager as root
hadoop_master | ERROR: but there is no YARN_RESOURCEMANAGER_USER defined. Aborting operation.
hadoop_master | Starting nodemanagers
hadoop_master | ERROR: Attempting to operate on yarn nodemanager as root
hadoop_master | ERROR: but there is no YARN_NODEMANAGER_USER defined. Aborting operation.
相同的安裝設定在hadoop 2.7.x是可以正常啟動的，但是hadoop 3.0.0就不行囉。
解決方法:
編輯${HADOOP_HOME}/etc/hadoop/hadoop-env.sh，加入下列設定:
export HDFS_DATANODE_USER=root
export HADOOP_SECURE_DN_USER=hdfs
export HDFS_NAMENODE_USER=root
export HDFS_SECONDARYNAMENODE_USER=root
export YARN_RESOURCEMANAGER_USER=root
由於叢集為測試使用，故使用者設定為root，正式環境請根據企業內部資安規範設置。
後記：
使用關鍵字：HDFS_NAMENODE_USER請google大神查完之後，找到了Hadoop 3.0.0-beta1官方文件的說明，看起來像是security相關設定，需要Hadoop神人來幫忙講解了！