Hadoop Interview Questions

What is Hadoop?
Hadoop is a framework that allows for distributed processing of large data sets across clusters of commodity computers using a simple programming model. Hadoop doesn’t have any expanding version. The charming yellow elephant you see is basically named after Doug’s son’s toy elephant!


What is the use of Hadoop?
Everyday a large amount of unstructured data is getting dumped into our machines. The major challenge is not to store large data sets in our systems but to retrieve and analyze the big data in the organizations, that too data present in different machines at different locations. In this situation a necessity for Hadoop arises. Hadoop has the ability to analyze the data present in different machines at different locations very quickly and in a very cost effective way. It uses the concept of MapReduce which enables it to divide the query into small parts and process them in parallel. This is also known as parallel computing.


What are the main characteristics of Hadoop framework?
Hadoop framework is written in Java. It is designed to solve problems that involve analyzing large data (e.g. petabytes). The programming model is based on Google’s MapReduce. The infrastructure is based on Google’s Big Data and Distributed File System. Hadoop handles large files/data throughput and supports data intensive distributed applications. Hadoop is scalable as more nodes can be easily added to it.


Which are the three modes in which Hadoop can be run?
The three modes in which Hadoop can be run are:
1. standalone (local) mode
2. Pseudo-distributed mode
3. Fully distributed mode


What are the features of Stand alone (local) mode?
In stand-alone mode there are no daemons, everything runs on a single JVM. It has no DFS and utilizes the local file system. Stand-alone mode is suitable only for running MapReduce programs during development. It is one of the most least used environments.


What are the features of Pseudo mode?
Pseudo mode is used both for development and in the QA environment. In the Pseudo mode all the daemons run on the same machine.


Can we call VMs as pseudos?
No, VMs are not pseudos because VM is something different and pesudo is very specific to Hadoop.


What are the features of Fully Distributed mode?
Fully Distributed mode is used in the production environment, where we have ‘n’ number of machines forming a Hadoop cluster. Hadoop daemons run on a cluster of machines.
There is one host onto which Namenode is running and another host on which datanode is running and then there are machines on which task tracker is running. We have separate masters and separate slaves in this distribution.


Does Hadoop follows the UNIX pattern?
Yes, Hadoop closely follows the UNIX pattern. Hadoop also has the ‘conf‘ directory as in the case of UNIX.


What is the Hadoop-core configuration?

Hadoop core is configured by two xml files:

1. hadoop-default.xml which was renamed to 2. hadoop-site.xml.

These files are written in xml format. We have certain properties in these xml files, which consist of name and value. But these files do not exist now.


What are the Hadoop configuration files at present?

There are 3 configuration files in Hadoop:

1. core-site.xml

2. hdfs-site.xml

3. mapred-site.xml

These files are located in the conf/ subdirectory.


What is a spill factor with respect to the RAM?
Spill factor is the size after which your files move to the temp file. Hadoop-temp directory is used for this.


Is fs.mapr.working.dir a single directory?
Yes, fs.mapr.working.dir it is just one directory.


Which are the three main hdfs-site.xml properties?
The three main hdfs-site.xml properties are:

1. dfs.name.dir which gives you the location on which metadata will be stored and where DFS is located – on disk or onto the remote.

2. dfs.data.dir which gives you the location where the data is going to be stored.

3. fs.checkpoint.dir which is for secondary Namenode.


How to come out of the insert mode?
To come out of the insert mode, press ESC, type :q (if you have not written anything) OR type :wq (if you have written anything in the file) and then press ENTER.


What is Cloudera and why it is used?
Cloudera is the distribution of Hadoop. It is a user created on VM by default. Cloudera belongs to Apache and is used for data processing.


What happens if you get a ‘connection refused java exception’ when you type hadoopfsck /?
It could mean that the Namenode is not working on your VM.


What does ‘jps’ command do?
This command checks whether your Namenode, datanode, task tracker, job tracker, etc are working or not.


How can I restart Namenode?

1. Click on stop-all.sh and then click on start-all.sh OR

2. Write sudo hdfs (press enter), su-hdfs (press enter), /etc/init.d/ha (press enter) and then /etc/init.d/hadoop-0.20-namenode start (press enter).


What is the full form of fsck?
Full form of fsck is File System Check.


How can we check whether Namenode is working or not?

To check whether Namenode is working or not, use the command /etc/init.d/hadoop-0.20-namenode status or as simple as jps.


What does the command mapred.job.tracker do?

The command mapred.job.tracker lists out which of your nodes is acting as a job tracker.


What does /etc /init.d do?

/etc /init.d specifies where daemons (services) are placed or to see the status of these daemons. It is very LINUX specific, and nothing to do with Hadoop.


How can we look for the Namenode in the browser?

If you have to look for Namenode in the browser, you don’t have to give localhost:8021, the port number to look for Namenode in the brower is 50070.


How to change from SU to Cloudera?

To change from SU to Cloudera just type exit.


Which files are used by the startup and shutdown commands?

Slaves and Masters are used by the startup and the shutdown commands.


What do slaves consist of?

Slaves consist of a list of hosts, one per line, that host datanode and task tracker servers.


What do masters consist of?

Masters contain a list of hosts, one per line, that are to host secondary namenode servers.


What does hadoop-env.sh do?

hadoop-env.sh provides the environment for Hadoop to run. JAVA_HOME is set over here.


Can we have multiple entries in the master files?

Yes, we can have multiple entries in the Master files.


Where is hadoop-env.sh file present?

hadoop-env.sh file is present in the conf location.


In Hadoop_PID_DIR, what does PID stands for?

PID stands for ‘Process ID’.


What does /var/hadoop/pids do?

It stores the PID.


What does hadoop-metrics.properties file do?

hadoop-metrics.properties is used for ‘Reporting‘ purposes. It controls the reporting for Hadoop. The default status is ‘not to report‘.


What are the network requirements for Hadoop?

The Hadoop core uses Shell (SSH) to launch the server processes on the slave nodes. It requires password-less SSH connection between the master and all the slaves and the secondary machines.


Do we need to give a password, even if the key is added in SSH?

Yes, password is still required even if the key is added in SSH.


What if a Namenode has no data?

If a Namenode has no data it is not a Namenode. Practically, Namenode will have some data.


What happens to job tracker when Namenode is down?

When Namenode is down, your cluster is OFF, this is because Namenode is the single point of failure in HDFS.


What happens to a Namenode, when job tracker is down?

When a job tracker is down, it will not be functional but Namenode will be present. So, cluster is accessible if Namenode is working, even if the job tracker is not working.


Can you give us some more details about SSH communication between Masters and the Slaves?

SSH is a password-less secure communication where data packets are sent across the slave. It has some format into which data is sent across. SSH is not only between masters and slaves but also between two hosts.


What is formatting of the DFS?

Just like we do for Windows, DFS is formatted for proper structuring. It is not usually done as it formats the Namenode too.


Does the HDFS client decide the input split or Namenode?

No, the Client does not decide. It is already specified in one of the configurations through which input split is already configured.


In Cloudera there is already a cluster, but if I want to form a cluster on Ubuntu can we do it?

Yes, you can go ahead with this! There are installation steps for creating a new cluster. You can uninstall your present cluster and install the new cluster.


Can we create a Hadoop cluster from scratch?
Yes we can do that also once we are familiar with the Hadoop environment.

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