Before setting up a Hadoop cluster, it is important to ensure that all prerequisites are addressed. Hadoop runs on the following operating systems:

Apache Hadoop requires the following software:

Apache Hadoop can be downloaded from http://www.apache.org/dyn/closer.cgi/Hadoop/common/. Make sure that you download and choose the correct release from different releases, that is, one that is a stable release, the latest beta/alpha release, or a legacy stable version. You can choose to download the package or download the source, compile it on your OS, and then install it. Using operating system package installer, install the Hadoop package. This software can be installed directly by using apt-get/dpkg for Ubuntu/Debian or rpm for Red Hat/Oracle Linux from the respective sites. In the case of a cluster setup, this software should be installed on all the machines.

Apache Hadoop uses ssh to run its scripts on different nodes, it is important to make this ssh login happen without any prompt for password. If you already have a key generated, then you can skip this step. To make ssh work without a password, run the following commands:

$ ssh-keygen -t dsa

You can also use RSA-based encryption algorithm (link to know about RSA: http://en.wikipedia.org/wiki/RSA_%28cryptosystem%29) instead of DSA (Digital Signature Algorithm) for your ssh authorization key creation. (For more information about differences between these two algorithms, visit http://security.stackexchange.com/questions/5096/rsa-vs-dsa-for-ssh-authentication-keys. Keep the default file for saving the key, and do not enter a passphrase. Once the key generation is successfully complete, the next step is to authorize the key by running the following command:

$ cat ~/.ssh/id_dsa.pub >> ~/.ssh/authorized_keys

This step will actually create an authorization key with ssh, bypassing the passphrase check as shown in the following screenshot:

Once this step is complete, you can ssh localhost to connect to your instance without password. If you already have a key generated, you will get a prompt to overwrite it; in such a case, you can choose to overwrite it or you can use the existing key and put it in the authorized_keys file.

Most of the Hadoop configuration is specified in the following configuration files, kept in the $HADOOP_HOME/etc/Hadoop folder of the installation. $HADOOP_HOME is the place where Apache Hadoop has been installed. If you have installed the software by using the pre-build package installer as the root user, the configuration can be found at /etc/Hadoop.

The file names marked in pink italicized letters will be modified while setting up your basic Hadoop cluster.

Now, let's start with the configuration of these files for the first Hadoop run. Open core-sites.xml, and add the following entry in it:

<configuration>
  <property>
    <name>fs.defaultFS</name>
    <value>hdfs://localhost:9000</value>
  </property>
</configuration>

This snippet tells the Hadoop framework to run inter-process communication on port 9000. Next, edit hdfs-site.xml and add the following entries:

<configuration>
  <property>
    <name>dfs.replication</name>
    <value>1</value>
  </property>
</configuration>

This tells HDFS to have the distributed file system's replication factor as 1. Later when you run Hadoop in the cluster configuration, you can change this replication count. The choice of replication factor varies from case to case, but if you are not sure about it, it is better to keep it as 3. This means that each document will have a replication of factor of 3.

Let's start looking at the MapReduce configuration. Some applications such as Apache HBase use only HDFS for storage, and they do not rely on the MapReduce framework. This means that all they require is the HDFS configuration, and the next configuration can be skipped.

Now, edit mapred-site.xml and add the following entries:

<configuration>
  <property>
    <name>mapreduce.framework.name</name>
    <value>yarn</value>
  </property>
</configuration>

This entry points to YARN as the MapReduce framework used. Further, modify yarn-site.xml with the following entries:

<configuration>
  <property>
    <name>yarn.nodemanager.aux-services</name>
    <value>mapreduce_shuffle</value>
  </property>
</configuration>

This entry enables YARN to use the ShuffleHandler service with nodemanager. Once the configuration is complete, we are good to start the Hadoop. Here are the default ports used by Apache Hadoop: