HDFS data blocks are distributed across local drives of all machines in a cluster while NAS data is stored on dedicated hardware.
Mention how Hadoop is different from other data processing tools?
In Hadoop, you can increase or decrease the number of mappers without worrying about the volume of data to be processed.
Hadoop will make 5 splits
1 split for 64K files
2 split for 65mb files
2 splits for 127mb files
Mention what is distributed cache in Hadoop?
Distributed cache in Hadoop is a facility provided by MapReduce framework. At the time of execution of the job, it is used to cache file. The Framework copies the necessary files to the slave node before the execution of any task at that node.
Explain how does Hadoop Classpath plays a vital role in stopping or starting in Hadoop daemons?
Classpath will consist of a list of directories containing jar files to stop or start daemons.
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For a job in Hadoop, is it possible to change the number of mappers to be created?
No, it is not possible to change the number of mappers to be created. The number of mappers is determined by the number of input splits.
Explain what is a sequence file in Hadoop?
To store binary key/value pairs, sequence file is used. Unlike regular compressed file, sequence file support splitting even when the data inside the file is compressed.
Explain what happens in textinformat ?
In textinputformat, each line in the text file is a record. Value is the content of the line while Key is the byte offset of the line. For instance, Key: longWritable, Value: text
Mention what are the main configuration parameters that user need to specify to run Mapreduce Job ?
The user of Mapreduce framework needs to specify
Job’s input locations in the distributed file system
Job’s output location in the distributed file system
Input format
Output format
Class containing the map function
Class containing the reduce function
JAR file containing the mapper, reducer and driver classes
Explain what is WebDAV in Hadoop?
To support editing and updating files WebDAV is a set of extensions to HTTP. On most operating system WebDAV shares can be mounted as filesystems , so it is possible to access HDFS as a standard filesystem by exposing HDFS over WebDAV.
Explain what is sqoop in Hadoop ?
To transfer the data between Relational database management (RDBMS) and Hadoop HDFS a tool is used known as Sqoop. Using Sqoop data can be transferred from RDMS like MySQL or Oracle into HDFS as well as exporting data from HDFS file to RDBMS
Explain how JobTracker schedules a task ?
The task tracker send out heartbeat messages to Jobtracker usually every few minutes to make sure that JobTracker is active and functioning. The message also informs JobTracker about the number of available slots, so the JobTracker can stay upto date with where in the cluster work can be delegated
Explain what is Sequencefileinputformat?
Sequencefileinputformat is used for reading files in sequence. It is a specific compressed binary file format which is optimized for passing data between the output of one MapReduce job to the input of some other MapReduce job.
Explain what does the conf.setMapper Class do ?
Conf.setMapperclass sets the mapper class and all the stuff related to map job such as reading data and generating a key-value pair out of the mapper
Explain what is Hadoop?
It is an open-source software framework for storing data and running applications on clusters of commodity hardware. It provides enormous processing power and massive storage for any type of data.
Mention Hadoop core components?
Hadoop core components include,
HDFS
MapReduce
Rack awareness is the way in which the namenode determines on how to place blocks based on the rack definitions.
Explain what is a Task Tracker in Hadoop?
A Task Tracker in Hadoop is a slave node daemon in the cluster that accepts tasks from a JobTracker. It also sends out the heartbeat messages to the JobTracker, every few minutes, to confirm that the JobTracker is still alive.
Mention what daemons run on a master node and slave nodes?
Daemons run on Master node is "NameNode"
Daemons run on each Slave nodes are “Task Tracker” and "Data"
Explain how can you debug Hadoop code?
The popular methods for debugging Hadoop code are:
By using web interface provided by Hadoop framework
By using Counters
Explain what is storage and compute nodes?
The storage node is the machine or computer where your file system resides to store the processing data
The compute node is the computer or machine where your actual business logic will be executed.
Mention what is the use of Context Object?
The Context Object enables the mapper to interact with the rest of the Hadoop
system. It includes configuration data for the job, as well as interfaces which allow it to emit output.
When Namenode is down what happens to job tracker?
Namenode is the single point of failure in HDFS so when Namenode is down your cluster will set off.
Explain how indexing in HDFS is done?
Hadoop has a unique way of indexing. Once the data is stored as per the block size, the HDFS will keep on storing the last part of the data which say where the next part of the data will be.
Explain is it possible to search for files using wildcards?
Yes, it is possible to search for files using wildcards.
List out Hadoop’s three configuration files?
The three configuration files are
core-site.xml
mapred-site.xml
hdfs-site.xml
Explain how can you check whether Namenode is working beside using the jps command?
Beside using the jps command, to check whether Namenode are working you can also use
/etc/init.d/hadoop-0.20-namenode status.
Explain what is “map” and what is "reducer" in Hadoop?
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.
In Hadoop, a reducer collects the output generated by the mapper, processes it, and creates a final output of its own.
In Hadoop, which file controls reporting in Hadoop?
In Hadoop, the hadoop-metrics.properties file controls reporting.
For using Hadoop list the network requirements?
For using Hadoop the list of network requirements are:
Password-less SSH connection
Secure Shell (SSH) for launching server processes
The next step after Mapper or MapTask is that the output of the Mapper are sorted, and partitions will be created for the output.
Mention what is the number of default partitioner in Hadoop?
In Hadoop, the default partitioner is a “Hash” Partitioner.
Explain what is the purpose of RecordReader in Hadoop?
In Hadoop, the RecordReader loads the data from its source and converts it into (key, value) pairs suitable for reading by the Mapper.
Mention what job does the conf class do?
Job conf class separate different jobs running on the same cluster. It does the job level settings such as declaring a job in a real environment.
Mention what is the Hadoop MapReduce APIs contract for a key and value class?
For a key and value class, there are two Hadoop MapReduce APIs contract
The value must be defining the org.apache.hadoop.io.Writable interface
The key must be defining the org.apache.hadoop.io.WritableComparable interface
Mention what are the three modes in which Hadoop can be run?
The three modes in which Hadoop can be run are
Pseudo distributed mode
Standalone (local) mode
Fully distributed mode
Mention what does the text input format do?
The text input format will create a line object that is an hexadecimal number. The value is considered as a whole line text while the key is considered as a line object. The mapper will receive the value as ‘text’ parameter while key as ‘longwriteable’ parameter.
What happens when a datanode fails ?
When a datanode fails
Jobtracker and namenode detect the failure
On the failed node all tasks are re-scheduled
Namenode replicates the users data to another node
Explain what is Speculative Execution?
In Hadoop during Speculative Execution a certain number of duplicate tasks are launched. On different slave node, multiple copies of same map or reduce task can be executed using Speculative Execution. In simple words, if a particular drive is taking long time to complete a task, Hadoop will create a duplicate task on another disk. Disk that finish the task first are retained and disks that do not finish first are killed.
Explain what are the basic parameters of a Mapper?
The basic parameters of a Mapper are
LongWritable and Text
Text and IntWritable
Explain what is the function of MapReducer partitioner?
The function of MapReducer partitioner is to make sure that all the value of a single key goes to the same reducer, eventually which helps evenly distribution of the map output over the reducers
Explain what is difference between an Input Split and HDFS Block?
Logical division of data is known as Split while physical division of data is known as HDFS Block
How Hadoop MapReduce works?
In MapReduce, during the map phase it counts the words in each document, while in the reduce phase it aggregates the data as per the document spanning the entire collection. During the map phase the input data is divided into splits for analysis by map tasks running in parallel across Hadoop framework.
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