MapReduce: Simplified Data Processing on Large Clusters

Steps in Map Reduce

Data Flow In MapReduce

The Algorithm

Inputs and Outputs (Java Perspective)

Input and Output types of a MapReduce job − (Input) <k1, v1> → map → <k2, v2> → reduce → <k3, v3>(Output).

MapReduce Input Output
Map <k1, v1> list (<k2, v2>)
Reduce <k2, list(v2)> list (<k3, v3>)


  • PayLoad − Applications implement the Map and the Reduce functions, and form the core of the job.
  • Mapper − Mapper maps the input key/value pairs to a set of intermediate key/value pair.
  • NamedNode − Node that manages the Hadoop Distributed File System (HDFS).
  • DataNode − Node where data is presented in advance before any processing takes place.
  • MasterNode − Node where JobTracker runs and which accepts job requests from clients.
  • SlaveNode − Node where Map and Reduce program runs.
  • JobTracker − Schedules jobs and tracks the assign jobs to Task tracker.
  • Task Tracker − Tracks the task and reports status to JobTracker.
  • Job − A program is an execution of a Mapper and Reducer across a dataset.
  • Task − An execution of a Mapper or a Reducer on a slice of data.
  • Task Attempt − A particular instance of an attempt to execute a task on a SlaveNode.

Example Scenario

Input Data

  • sample.txt
1979 23 23 2 43 24 25 26 26 26 26 25 26 25
1980 26 27 28 28 28 30 31 31 31 30 30 30 29
1981 31 32 32 32 33 34 35 36 36 34 34 34 34
1984 39 38 39 39 39 41 42 43 40 39 38 38 40
1985 38 39 39 39 39 41 41 41 00 40 39 39 45

Example Program


Compilation and Execution of Process Units Program

  1. mkdir units

  2. /home/hadoop/hadoop-core-1.2.1.jar

  3. javac -classpath hadoop-core-1.2.1.jar -d units

    • jar -cvf units.jar -C units/ .
  4. hadoop fs -mkdir /input_dir

  5. hadoop fs -put /home/hadoop/sample.txt /input_dir

  6. hadoop fs -ls /input_dir/

  7. hadoop jar units.jar t5750.hadoop.ProcessUnits /input_dir /output_dir

  8. hadoop fs -ls /output_dir/

  9. hadoop fs -cat /output_dir/part-00000

    1981    34
    1984    40
    1985    45
  10. hadoop fs -cat /output_dir/part-00000 | hadoop fs -get /output_dir /home/hadoop

Important Commands

Usage − hadoop [--config confdir] COMMAND

Option Description
namenode -format Formats the DFS filesystem.
secondarynamenode Runs the DFS secondary namenode.
namenode Runs the DFS namenode.
datanode Runs a DFS datanode.
dfsadmin Runs a DFS admin client.
mradmin Runs a Map-Reduce admin client.
fsck Runs a DFS filesystem checking utility.
fs Runs a generic filesystem user client.
balancer Runs a cluster balancing utility.
oiv Applies the offline fsimage viewer to an fsimage.
fetchdt Fetches a delegation token from the NameNode.
jobtracker Runs the MapReduce job Tracker node.
pipes Runs a Pipes job.
tasktracker Runs a MapReduce task Tracker node.
historyserver Runs job history servers as a standalone daemon.
job Manipulates the MapReduce jobs.
queue Gets information regarding JobQueues.
version Prints the version.
jar <jar> Runs a jar file.
distcp <srcurl> <desturl> Copies file or directories recursively.
distcp2 <srcurl> <desturl> DistCp version 2.
archive -archiveName NAME -p <parent path> <src>* <dest> Creates a hadoop archive.
classpath Prints the class path needed to get the Hadoop jar and the required libraries.
daemonlog Get/Set the log level for each daemon

How to Interact with MapReduce Jobs

Usage − hadoop job [GENERIC_OPTIONS]

-submit <job-file> Submits the job.
-status <job-id> Prints the map and reduce completion percentage and all job counters.
-counter <job-id> <group-name> <countername> Prints the counter value.
-kill <job-id> Kills the job.
-events <job-id> <fromevent-#> <#-of-events> Prints the events' details received by jobtracker for the given range.
-history [all] <jobOutputDir> - history < jobOutputDir> Prints job details, failed and killed tip details. More details about the job such as successful tasks and task attempts made for each task can be viewed by specifying the [all] option.
-list[all] Displays all jobs. -list displays only jobs which are yet to complete.
-kill-task <task-id> Kills the task. Killed tasks are NOT counted against failed attempts.
-fail-task <task-id> Fails the task. Failed tasks are counted against failed attempts.
-set-priority <job-id> <priority> Changes the priority of the job. Allowed priority values are VERY_HIGH, HIGH, NORMAL, LOW, VERY_LOW

To see the status of job

hadoop job -status <JOB-ID>
hadoop job -status job_1557970197535_0001

To see the history of job output-dir

hadoop job -history <DIR-NAME>
hadoop job -history job_1557970197535_0001

To kill the job

hadoop job -kill <JOB-ID>
hadoop job -kill job_1557970197535_0001

Word Count Example

Steps to execute MapReduce word count example

mkdir word-count
vi ~/word-count/word-count-data.txt
HDFS is a storage unit of Hadoop
MapReduce is a processing tool of Hadoop
hdfs dfs -mkdir /word-count
hdfs dfs -put ~/word-count/word-count-data.txt /word-count
WordCountMapper, WordCountReducer, WordCountRunner
# IDE-->Gradle-->Tasks-->build-->jar
hadoop jar ~/word-count/hadoop-demos.jar t5750.hadoop.mapred.WordCountRunner /word-count /word-count-output
# Browsing HDFS:
hdfs dfs -cat /word-count-output/part-00000

MapReduce Char Count Example

Steps to execute MapReduce char count example

mkdir char-count
vi ~/char-count/char-count-info.txt
hdfs dfs -mkdir /char-count
hdfs dfs -put ~/char-count/char-count-info.txt /char-count
CharCountMapper, CharCountReducer, CharCountRunner
# IDE-->Gradle-->Tasks-->build-->jar
hadoop jar ~/char-count/hadoop-demos.jar t5750.hadoop.mapred.CharCountRunner /char-count /char-count-output
hdfs dfs -cat /char-count-output/part-00000