MapReduce Interview Questions

What does MapReduce mean?

MapReduce is a programming framework for processing large sets of data and big data over thousands of servers within a Hadoop cluster. The MapReduce concept is the same as the data processing systems across the cluster. The word MapReduce refers to two key processes of the Hadoop program operates. The first job is the map() that converts a set of data into another by breaking down a single element into key or value pairs (tuples). Then the reduce() job comes into the picture in which the map jobs output, i.e., the tuples are used as the input in reduce() job, and they are combined as a smaller set of tuples. Map job is performed before every reduce() job.

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How does MapReduce work?

MapReduce is divided into two phases. The first one is the map and the second is reduce.

Map: This phase involves sorting the data or counting words. 

Reduce: This phase involves reducing the data and then aggregating them.

As a result, the data is initially divided for analysis purposes.

What are the major components of MapReduce?

The following are the key components of MapReduce:

Main class: It involves giving the main parameters to the job, like giving various data files for sorting. 

Mapper Class: In this class, mainly mapping is done. The map method will be executed. 

Reducer Class: Aggregate data is pushed into the reducer class. There is a reduction in data in this class.

List three advantages of MapReduce.

MapReduce has three significant advantages:

Extremely scalable: It Stores and distributes large data sets on thousands of servers.

Cost-effective: It allows us to store and process data at affordable prices.

Secure: It enables only the authorized users for operating the data and includes HDFS and HBase security.

What is meant by Inputformat in Hadoop?

An important feature in the Mapreduce framework is Inputformat. Inputformat specifies the input requirements for a job. It does the following actions:

• It validates the job's input specification.
• It divides the input files into logical instances referred to as InputSplit. Each of the divided files is subsequently assigned to an individual mapper.
• It Provides the RecordReader implementation for extracting input records from the instances mentioned above for other Mapper processing.

Explain job scheduling within JobTracker.

JobTracker contacts the NameNode to find out where the data is located and then submits the work to the TaskTracker node. TaskTracker plays an important role because it alerts the JobTracker of any work failure. It is, in fact, referred to as a heartbeat reporter reassuring job tracker that it is alive. The JobTracker will be responsible for the actions, as it can submit the job again or mark a particular record as unreliable or can blacklist it.

What do you mean by SequenceFileInputFormat?

SequenceFileInputFormat is a binary output file format (that is compressed) for reading sequenced files and extending the FileInputFormat. The SequenceFileInputFormat transfers data between the output-input phases of the MapReduce tasks (i.e., in between the output from a MapReduce job and input from another MapReduce job).

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Explain Shuffling and Sorting in MapReduce.

Shuffling and Sorting are the two important processes that work parallelly with the Mapper and reducer.

Shuffling: The process in which data is transferred to the reducer from Mapper is called as Shuffling. The reducer must continue its job further as the process of shuffling is used as the input to the reduce task.

Sorting: In Mapreduce, the output key-value pairs across the mapper phase and reduce phase are sorted automatically before they are moved to the reducer. This function is useful for programs in which you have to sort at certain steps. It will save the overall time of the programmer.

What is JobConf in MapReduce?

This is the main interface for defining a map-reduce job within the Hadoop to perform the job. JobConf determines combiner, Mapper, Reducer, partitioner, InputFormat, OutputFormat implementations and more. They are described in the Mapreduce online reference guide and in the Mapreduce community.

) What is MapReduce Combiner?

MapReduce Combiner is also referred to as semi-reducer. It is an optional class for combining the map of records with the same key. The primary function of the combiner is to accept the inputs from the Map class and pass these key-value pairs to the reducer class.

Intermediate level MapReduce Interview Questions:

) Explain RecordReader in MapReduce.

RecordReader is used for reading key or value pairs from InputSplit converting the byte-oriented view and sending the record-oriented view to the Mapper.

) What is Chain Mapper and Identity Mapper?

Chain Mapper:  Chain Mapper is a simple Mapper class implemented by chain operations on a set of Mapper classes in a single map task. Here the first Mapper's output will be the input to the second Mapper, and the second Mapper's output will become the input to the third Mapper, and this process continues till the last Mapper. org.apache.hadoop.mapreduce.lib.ChainMapper is the class name.

Identity Mapper: Identity Mapper is Hadoop's default mapper class. When another Mapper class is not set, Identity Mapper will be executed. It will only write the input data to output and will not perform any computations or calculations on the input data. org.apache.hadoop.mapreduce.lib.IdentityMapper is the class name.

) What do you mean by partitioning?

Partitioning is the process of identifying the instance of reducer that will be used for providing mapper output. Mapper, before emitting the key-value pair for the reducer, Mapper identifies the reducer as the receiver of the output of the Mapper. Any key, regardless of which Mapper produced this, has to be with the same reducer.

) What are the configuration parameters required to perform the job of the MapReduce framework?

The following parameters must be specified by the user:

• An input design format.
• An output design format.
• The job or data input location is required to be determined in the file system.
• The data output location must also be determined within the system.
• Defining the mapper function's specific class
• Defining the reducer function's specific class
• JAR file that is made up of all mapper and reducer classes.

) What is text Input format?

The only default format of text files or input data is the text input format. Files are broken in the text input format. The line of the text refers to the value, while the key refers to the position. Both of these are the major components in the data files.

Related Article: MapReduce In Big Data

) Explain the differences between combiner and reducer.

Any local tasks to reduce local data files are carried out using a combiner. It works primarily on map output. Similar to the reducer, it also generates the output for the input of the reducer. Combiner also has other uses, such as it is frequently used as the network optimization job, particularly when the number of outputs increases by the Map generator. Combiners differ from the reducer in a number of ways. A reducer is restricted; however, a combiner has limitations such as input or output data, and the values should be the same as the mapper output data. A combiner may work with the commutative function. For example, it may operate on the subsets of the keys and values of the data. A combiner will get the input from a single mapper while the reducer gets the input from a number of mappers.

) What does the term heartbeat mean that is used on HDFS?

HDFS refers to Hadoop Distributed File System. It is the most critical component in the Hadoop Architecture and will be responsible for the data storage. The signal that is used in HDFS is referred to as the heartbeat. The signal is primarily transmitted between two kinds of nodes, which are data and name nodes. It occurs in between task tracker and job tracker. If the signal does not work properly and if there are problems with both nodes or trackers, it is regarded as having a poor heartbeat.

) What are the consequences of the data node failure?

Consequences of a data node failure include:

• All tasks will be rescheduled as the nodes failed do not allow data to pass by the mapping and reducing processes, and therefore it is rescheduled to complete the process. 
• The data node failure is primarily determined by the other kind of node, the name mode, as well as the job tracker. 
• The data is replicated at a different node through the name node. The main reason for this is the completion of the process.

) What is Speculative execution?

Speculative execution is a kind of feature that makes it possible to launch multiple tasks on different types of nodes. Usually, duplicate copies of the task are created with the help of the function if a task takes more time to complete. Sometimes even certain multiple copies as well as done through speculative execution.

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) How do Identity mapper and identity reducer differ?

The identity mapper is related to the default mapper class, and the identity reducer is related to the default reducer class. If the mapper number is not defined during the work process, it is referred to as the identity mapper. When a reducer class is not defined, it is referred to as an identity reducer. As a result, this class transfers key values into the output directory.

MapReduce Interview Questions for Experienced:

) What is the difference between MapReduce and PIG?

PIG is essentially the dataflow language which handles the data flow from source to source. It manages and contributes to the compression of the data storage system. Pig reorganizes the steps to quicker and more efficient processing. PIG primarily handles MapReduce output data. Certain features of the MapReduce processing are included in the PIG processing. Functions include data grouping, ordering and counting. MapReduce is essentially the framework to write code for developers. It is a paradigm of data processing which separates the application of two kinds of developers, the people who write it and the people who scale it.

) Where map-reduce is not recommended?

Mapreduce is not advisable for iterative processing. This means repeating the output over and over again. For processing the Mapreduce job series, MapReduce is not suitable. Every job persists the data into the local drive and then loads again to the other job. It will be a costly operation. So it is not recommended.

) What do you mean by Outputcommitter?

OutPutCommitter depicts the commit of the MapReduce task. FileOutputCommitter is the current default class of OutputCommitter in MapReduce. Following are the operations performed by OutPutCommitter.

• Create a temporary output folder for the task during the initialization.
• Next, it clears the job as it deletes the temporary output directory after completion of the job.
• Configures temporary output for the task.
• Decides if a task needs to commit. The commit will apply if necessary.
• JobCleanup, TaskCleanup and JobSetup are important tasks while output commit.

) What is JobTracker?

• JobTracker is a service which is used to handle MapReduce tasks within a cluster. The following are the functions the JobTracker does:
• It accepts the jobs that are submitted by the client application.
• It communicates the NameNode regarding the location of the data.
• It finds out the nearby or available TaskTracker nodes.
• It Submits the works to the selected nodes.
• It will update the job status once completed.
• When a TaskTracker node reports a failure, JobTracker decides that the next steps should be followed.
• When the JobTracker is unsuccessful, all execution work will be stopped.

) What types of InputFormat are available on MapReduce?

InputFormat is a MapReduce feature that sets input specifications to a job. There are 8 different types of InputFormat in MapReduce. They are:

• FileInputFormat
• TextInputFormat
• DBInputFormat
• NLineInputFormat
• SequenceFileInputFormat
• KeyValueTextInputFormat
• SequenceFileAsTextInputFormat
• SequenceFileAsBinaryInputFormat

) What are the differences between the HDFS block and InputSplit?

HDFS block: It is responsible for dividing the data into specific physical divisions.

Inputsplit: It is responsible for the logical splitting of Input files.

The InputSplit will also be able to control the number of mappers, while the size of the splits is user-specified. In HDFS, the HDFS block size is fixed as 64MB for 1GB data, 1GB/64MB = 16 blocks/splits. However, if the user does not set the size of the input split, then it will assume the size of the default HDFS block.

) What is NameNode?

A NameNode in Hadoop is called the node, where Hadoop is able to store all information about the location of the file in the Hadoop Distributed File System. Simply put, a NameNode is a central element or feature of the HDFS filesystem. It is responsible for retaining the record of all files into the file system and also monitoring file data across the cluster or several machines.

) what are the various job control options provided by MapReduce.

MapReduce framework supports chained operations. So the output of one map job is used as the input of another map job. That is why there is a need for job controls to govern and work with these complex jobs. Various job control options are as follows:

Job.submit(): It is used for submitting the job to a cluster and return immediately.

Job.waitforCompletion(boolean): It is responsible for submitting the work to the cluster and wait until it is finished.

) How many Daemon processes may be executed on a Hadoop system?

) What is WebDAV?

WebDAV is a set of HTTP extensions that offer extended support to edit and update files. On the majority of operating systems, WebDAV shares may be mounted as file systems. HDFS can thus be accessed as a standard filesystem by exhibiting HDFS on WebDAV.

Conclusion: 

We hope you will find this blog helpful in your preparation for your interview. We attempted to cover the basic, intermediate and advanced frequently asked interview questions of MapReduce. Do not hesitate to put your questions in the comments section below. We'll try to respond as best we can.