November 10, 2018

Sreekanth B

Amadeus IT Group Most Frequently Asked Latest Hadoop Interview Questions Answers

What are the different types of Znodes?

There are 2 types of Znodes namely- Ephemeral and Sequential Znodes.

The Znodes that get destroyed as soon as the client that created it disconnects are referred to as Ephemeral Znodes.
Sequential Znode is the one in which sequential number is chosen by the ZooKeeper ensemble and is pre-fixed when the client assigns name to the znode.

What are watches?

Client disconnection might be troublesome problem especially when we need to keep a track on the state of Znodes at regular intervals. ZooKeeper has an event system referred to as watch which can be set on Znode to trigger an event whenever it is removed, altered or any new children are created below it.

What is a checkpoint?

In brief, “Checkpointing” is a process that takes an FsImage, edit log and compacts them into a new FsImage. Thus, instead of replaying an edit log, the NameNode can load the final in-memory state directly from the FsImage. This is a far more efficient operation and reduces NameNode startup time. Checkpointing is performed by Secondary NameNode.

How is HDFS fault tolerant?

When data is stored over HDFS, NameNode replicates the data to several DataNode. The default replication factor is 3. You can change the configuration factor as per your need. If a DataNode goes down, the NameNode will automatically copy the data to another node from the replicas and make the data available. This provides fault tolerance in HDFS.

Can NameNode and DataNode be a commodity hardware?

The smart answer to this question would be, DataNodes are commodity hardware like personal computers and laptops as it stores data and are required in a large number. But from your experience, you can tell that, NameNode is the master node and it stores metadata about all the blocks stored in HDFS. It requires high memory (RAM) space, so NameNode needs to be a high-end machine with good memory space.
Amadeus IT Group Most Frequently Asked Latest Hadoop Interview Questions Answers
Amadeus IT Group Most Frequently Asked Latest Hadoop Interview Questions Answers

Why do we use HDFS for applications having large data sets and not when there are a lot of small files?

HDFS is more suitable for large amounts of data sets in a single file as compared to small amount of data spread across multiple files. As you know, the NameNode stores the metadata information regarding the file system in the RAM. Therefore, the amount of memory produces a limit to the number of files in my HDFS file system. In other words, too many files will lead to the generation of too much metadata. And, storing these metadata in the RAM will become a challenge. As a thumb rule, metadata for a file, block or directory takes 150 bytes.

How do you define “block” in HDFS? What is the default block size in Hadoop 1 and in Hadoop 2? Can it be changed?

Blocks are the nothing but the smallest continuous location on your hard drive where data is stored. HDFS stores each as blocks, and distribute it across the Hadoop cluster. Files in HDFS are broken down into block-sized chunks, which are stored as independent units.

Hadoop 1 default block size: 64 MB
Hadoop 2 default block size:  128 MB
Yes, blocks can be configured. The dfs.block.size parameter can be used in the hdfs-site.xml file to set the size of a block in a Hadoop environment.

What does ‘jps’ command do?

The ‘jps’ command helps us to check if the Hadoop daemons are running or not. It shows all the Hadoop daemons i.e namenode, datanode, resourcemanager, nodemanager etc. that are running on the machine.

How do you define “Rack Awareness” in Hadoop?

Rack Awareness is the algorithm in which the “NameNode” decides how blocks and their replicas are placed, based on rack definitions to minimize network traffic between “DataNodes” within the same rack. Let’s say we consider replication factor 3 (default), the policy is that “for every block of data, two copies will exist in one rack, third copy in a different rack”. This rule is known as the “Replica Placement Policy”.

What is “speculative execution” in Hadoop?

If a node appears to be executing a task slower, the master node can redundantly execute another instance of the same task on another node. Then, the task which finishes first will be accepted and the other one is killed. This process is called “speculative execution”.

How can you transfer data from Hive to HDFS?

By writing the query:

hive> insert overwrite directory '/' select * from emp;
You can write your query for the data you want to import from Hive to HDFS. The output you receive will be stored in part files in the specified HDFS path.

What is Hadoop Map Reduce ?

For processing large data sets in parallel across a hadoop cluster, Hadoop MapReduce framework is used.  Data analysis uses a two-step map and reduce process.

Explain what combiners is and when you should use a combiner in a MapReduce Job?

To increase the efficiency of MapReduce Program, Combiners are used.  The amount of data can be reduced with the help of combiner’s that need to be transferred across to the reducers. If the operation performed is commutative and associative you can use your reducer code as a combiner.  The execution of combiner is not guaranteed in Hadoop

Explain what is JobTracker in Hadoop? What are the actions followed by Hadoop?

In Hadoop for submitting and tracking MapReduce jobs,  JobTracker is used. Job tracker run on its own JVM process

Job Tracker performs following actions in Hadoop

Client application submit jobs to the job tracker
JobTracker communicates to the Namemode to determine data location
Near the data or with available slots JobTracker locates TaskTracker nodes
On chosen TaskTracker Nodes, it submits the work
When a task fails, Job tracker notify and decides what to do then.
The TaskTracker nodes are monitored by JobTracker

Explain what is heartbeat in HDFS?

Heartbeat is referred to a signal used between a data node and Name node, and between task tracker and job tracker, if the Name node or job tracker does not respond to the signal, then it is considered there is some issues with data node or task tracker

What is NameNode in Hadoop?

NameNode in Hadoop is where Hadoop stores all the file location information in HDFS. It is the master node on which job tracker runs and consists of metadata.

Mention what are the data components used by Hadoop?

Data components used by Hadoop are

Pig
Hive

Mention what is the data storage component used by Hadoop?

The data storage component used by Hadoop is HBase.

Mention what are the most common input formats defined in Hadoop?

The most common input formats defined in Hadoop are;

TextInputFormat
KeyValueInputFormat
SequenceFileInputFormat

In Hadoop what is InputSplit?

It splits input files into chunks and assign each split to a mapper for processing.

For a Hadoop job, how will you write a custom partitioner?

You write a custom partitioner for a Hadoop job, you follow the following path

Create a new class that extends Partitioner Class
Override method getPartition
In the wrapper that runs the MapReduce
Add the custom partitioner to the job by using method set Partitioner Class or – add the custom partitioner to the job as a config file

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