Hadoop Interview Questions

Q1. What is a Hadoop framework?

Ans: Hadoop is a open source framework which is written in java by apche software foundation. Thisframework is used to wirite software application which requires to process vast amount of data (It could handlemulti tera bytes of data). It works in-paralle on large clusters which could have 1000 of computers (Nodes) on theclusters. It also process data very reliably and fault-tolerant manner. See the below image how does it looks.

Q2. On What concept the Hadoop framework works?

Ans : It works on MapReduce, and it is devised by the Google.

Q3. What is MapReduce ?

Ans: Map reduce is an algorithm or concept to process Huge amount of data in a faster way. As per its name youcan divide it Map and Reduce. The main MapReduce job usually splits the input data-set into independent chunks.

· (Big data sets in the multiple small datasets)

· MapTask: will process these chunks in a completely parallel manner (One node canprocess one or more chunks).

· The framework sorts the outputs of the maps.

· Reduce Task : And the above output will be the input for the reducetasks, produces thefinal result.

Your business logic would be written in the MappedTask and ReducedTask. Typically both the input and the output ofthe job are stored in a file-system (Not database). The framework takes care of scheduling tasks, monitoring themand re-executes the failed tasks.

Q4. What is compute and Storage nodes?


Compute Node: This is the computer or machine where your actual business logic will be executed.

Storage Node: This is the computer or machine where your file system reside to store the processing data.

In most of the cases compute node and storage node would be the same machine.

Q5. How does master slave architecture in the Hadoop?

Ans: The MapReduce framework consists of a single master JobTracker and multiple slaves, each cluster-node will have one TaskskTracker. The master is responsible for scheduling the jobs' component tasks on the slaves,monitoring them and re-executing the failed tasks. The slaves execute the tasks as directed by the master.

Q6. How does an Hadoop application look like or their basic components?

Ans: Minimally an Hadoop application would have following components.

· Input location of data

· Output location of processed data.

· A map task.

· A reduced task.

· Job configuration

The Hadoop job client then submits the job (jar/executable etc.) and configuration to the JobTracker which thenassumes the responsibility of distributing the software/configuration to the slaves, scheduling tasks and monitoringthem, providing status and diagnostic information to the job-client.

Q7. Explain how input and output data format of the Hadoop framework?

Ans: The MapReduce framework operates exclusively on pairs, that is, the framework views the input to the jobas a set of pairs and produces a set of pairs as the output of the job, conceivably of different types. See the flow mentioned below

(input) -> map -> -> combine/sorting -> -> reduce -> (output)

Q8. What are the restriction to the key and value class ?

Ans: The key and value classes have to be serialized by the framework. To make them serializable Hadoop provides aWritable interface. As you know from the java itself that the key of the Map should be comparable, hence the key has toimplement one more interface WritableComparable.

Q9. Explain the WordCount implementation via Hadoop framework ?

Ans: We will count the words in all the input file flow as below

· input

Assume there are two files each having a sentence HelloWorld Hello World (In file 1) Hello WorldHello World (In file 2)

· Mapper : There would be each mapper for the a file

For the given sample input the first map output:

< Hello, 1>

< World, 1>

< Hello, 1>

< World, 1>

The second map output:

< Hello, 1>

< World, 1>

< Hello, 1>

< World, 1>

· Combiner/Sorting (This is done for each individual map)

So output looks like this

The output of the first map:

< Hello, 2>

< World, 2>

The output of the second map:

< Hello, 2>

< World, 2>

· Reducer :

· Output

It sums up the above output and generates the output as below

< Hello, 4>

< World, 4>

Final output would look like

Hello 4 times

World 4 times

Q10. Which interface needs to be implemented to create Mapper and Reducer for the Hadoop?

Ans: org.apache.hadoop.mapreduce.Mapperorg.apache.hadoop.mapreduce.Reducer

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