Tuesday, August 23, 2016

Hadoop - How to setup a Hadoop Cluster

Below is a step-by-step guide which I had used to setup a Hadoop Cluster


3 VMs involved:

1) NameNode, ResourceManager - Host name: NameNode.net
2) DataNode 1 - Host name: DataNode1.net
3) DataNode 2 - Host name: DataNode2.net


1) You could create a new Hadoop user or use an existing user. But make sure the user have access to the Hadoop installation in ALL nodes

2) Install JAVA. Refer here for a good version. In this guide, Java is installed at /usr/java/latest

3) Download a stable version of Hadoop from Apache Mirrors

This guide is based on Hadoop 2.7.1 and assume that we had create a user call hadoop

Setup Passphaseless SSH from NameNode to all Nodes.

1) Run the command


This command will ask you a set of questions and accepting the default is fine. Eventually, it will create a set of private key (id_rsa) and public key (id_rsa.pub) at the user directory (/home/hadoop/.ssh)

2) Copy the public key to all Nodes with the following

ssh-copy-id -i /home/hadoop/.ssh/id_rsa.pub NameNode.net
ssh-copy-id -i /home/hadoop/.ssh/id_rsa.pub DataNode1.net
ssh-copy-id -i /home/hadoop/.ssh/id_rsa.pub DataNode2.net

3) Test the passphaseless SSH connection from NameNode with

ssh (hostname)

Install Hadoop in all Node

1) With the downloaded Hadoop distribution. Unzip it to a location where the Hadoop user had access

For this guide, I had create a /usr/local/hadoop and un-tar the distribution at this folder. The full path of Hadoop installation is /usr/local/hadoop/hadoop-2.7.1

Setup Environment Variables

1) It is best that Hadoop Variables are exported to the environment when user log in. To do so, run the command at the NameNode

sudo vi /etc/profile.d/hadoop.sh

2) Add the following in /etc/profile.d/hadoop.sh

export JAVA_HOME=/usr/java/latest
export HADOOP_HOME=/usr/hadoop
export HADOOP_OPTS="-Djava.library.path=$HADOOP_HOME/lib/native"
3) Source this file or re-login to setup the environment.

4) (OPTIONAL) Set up the above for all Nodes.

Setup NameNode & ResourceManager

1) Make a directory to hold NameNode data

mkdir /usr/local/hadoop/hdfs_namenode

2) Setup $HADOOP_HOME/etc/hadoop/hdfs-site.xml

Note: dfs.datanode.data.dir value must be a URI

3) Setup $HADOOP_HOME/etc/hadoop/core-site.xml

4) (OPTIONAL) Setup $HADOOP_HOME/etc/hadoop/mapred-site.xml (We are using NameNode as ResourceManager)

5) (OPTIONAL) Setup $HADOOP_HOME/etc/hadoop/yarn-site.xml (We are using NameNode as ResourceManager)

6) Setup $HADOOP_HOME/etc/hadoop/slaves

First, remove localhost from the file, then add the following

Setup DataNodes

1) Make a directory to hold DataNode data

mkdir /usr/local/hadoop/hdfs_datanode

2) Setup $HADOOP_HOME/etc/hadoop/hdfs-site.xml

Note: dfs.datanode.data.dir value must be a URI

3) Setup $HADOOP_HOME/etc/hadoop/core-site.xml

Format NameNode

The above setting should be enough to set up the Hadoop cluster. Next, for the first time, you will need to format the NameNode. Use the following command to format the NameNode

hdfs namenode -format

Example output is

Note: the same command can be used to reformat your existing NameNode. But remember to clean up your datanodes hdfs folder as well.

Start NameNode

You can start Hadoop with the given script


Example output is

Stop NameNode

You can stop Hadoop with the given script


Example output is

Start ResourceManager

You can start the ResourceManager, in this case, Yarn, with the given script


Example output is

Stop ResourceManager

You can stop the ResourceManager, in this case, Yarn, with the given script


Example output is

Show status of Hadoop

You can use the following command to show status of Hadoop


Example output is

Complete Testing

You can also do the following to perform a complete test to ensure Hadoop is running fine.

You could access the Hadoop Resource Manager information at http://NameNode_hostname:8088

You could also access the Hadoop cluster summary at http://NameNode_hostname:50070. You should be able to see the number of datanodes being setup for the cluster.


1. http://www.server-world.info/en/note?os=CentOS_7&p=hadoop
2. http://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-common/ClusterSetup.html

Tuesday, August 9, 2016

JAVA - _JAVA_OPTIONS and JAVA_TOOL_OPTIONS environment variable

JAVA_TOOL_OPTIONS and _JAVA_OPTIONS are 2 useful environment variables which allow user to set JVM options in the form of environment variables, rather than setting it at the command line. But, they have slight differences

1. Precedence - From my testing, the precedence (order of evaluation) is


With this, there is different use-case for _JAVA_OPTIONS and JAVA_TOOL_OPTIONS

For _JAVA_OPTIONS, you could use it to overwrite the JVM options which has been defined in the command line.

For JAVA_TOOL_OPTIONS, you could use it to put additional JVM options for the predefined command line.

2. Documentation - JAVA_TOOL_OPTIONS is well documented but _JAVA_OPTIONS. So, _JAVA_OPTIONS may not be officially supported.

3. Support - _JAVA_OPTIONS is Oracle specific. The IBM Java equivalent will be IBM_JAVA_OPTIONS. JAVA_TOOL_OPTIONS is platform independent.


3. http://stackoverflow.com/questions/28327620/difference-between-java-options-java-tool-options-and-java-opts