Hadoop Interview Questions and Answers

Ques 21. What is "map" and what is "reducer" in Hadoop?

Map: In Hadoop, a map is a phase in HDFS query solving. A map reads data from an input location and outputs a key-value pair according to the input type.

Reducer: In Hadoop, a reducer collects the output generated by the mapper, processes it, and creates a final output of its own.

Ques 22. What is shuffling in MapReduce?

Shuffling is a process which is used to perform the sorting and transfer the map outputs to the reducer as input.

Ques 23. What is NameNode in Hadoop?

NameNode is a node, where Hadoop stores all the file location information in HDFS (Hadoop Distributed File System). We can say that NameNode is the centerpiece of an HDFS file system which is responsible for keeping the record of all the files in the file system, and tracks the file data across the cluster or multiple machines.

Ques 24. What is heartbeat in HDFS?

Heartbeat is a signal which is used between a data node and name node, and between task tracker and job tracker. If the name node or job tracker doesn't respond to the signal then it is considered that there is some issue with data node or task tracker.

Ques 25. How is indexing done in HDFS?

There is a very unique way of indexing in Hadoop. Once the data is stored as per the block size, the HDFS will keep on storing the last part of the data which specifies the location of the next part of the data.