Month: March 2017

Hadoop MapReduce Interview Questions and Answers Explain the usage of Context Object. Context Object is used to help the mapper interact with other Hadoop systems. Context Object can be used for updating counters, to report the progress and to provide any application level status updates. ContextObject has the configuration details for the job and also interfaces, that helps it to generating the output. What are the core methods of a Reducer? The 3 core methods of a reducer are – 1)setup () – This method of the reducer is used for configuring various parameters like the input data size, distributed cache, heap size, etc. Function Definition- public void setup (context) 2)reduce () it is heart of the reducer which is called once per key with the associated reduce task. Function Definition -...

What is a block and block scanner in HDFS? Block – The minimum amount of data that can be read or written is generally referred to as a “block” in HDFS. The default size of a block in HDFS is 64MB. Block Scanner – Block Scanner tracks the list of blocks present on a DataNode and verifies them to find any kind of checksum errors. Block Scanners use a throttling mechanism to reserve disk bandwidth on the datanode. Explain the difference between NameNode, Backup Node and Checkpoint NameNode. NameNode: NameNode is at the heart of the HDFS file system which manages the metadata i.e. the data of the files is not stored on the NameNode but rather it has the directory tree of all the files present in the HDFS file system on a hadoop cluster. NameNode uses two files for the namespace- f...