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Exercise 2 Programming MapReduce Applications in Java

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ISIT312 Big Data Management
Exercise 2
Programming MapReduce Applications in Java
In this exercise, you will get familiar with programming of simple MapReduce applications.
Be careful when copying the Linux commands in this document to your working Terminal, because
it is error-prone. Maybe you should type those commands by yourself.
Prologue
Login to your system and start VirtualBox.
When ready start a virtual machine ISIT312-BigDataVM-07-SEP-2020.
When the virtual machine is running, leftclick at Terminal icon to open Terminal window.
In this practice you can either use Zeppelin interface described in the previous practice or use can
use command line interface in Terminal window. If you plan to use Zeppelin then perform the
following actions.
(1) Start Zeppelin and create a new notebook named, say, Lab2.
(2) Import Lab1 notebook created and saved in the previous exercise.
(3) Copy the statements processed in Lab1 into Lab2 notebook to Start the services of HDFS,
YARN, and Job History Server.
If you plan to use command line in terminal window then type all commands at $ prompt in terminal
window.
(1) How to start Hadoop ?
Open a new Terminal window or Zeppelin paragraph and start all Hadoop processes in the following
way.
$HADOOP_HOME/sbin/start-all.sh
jps
When ready minimize the Terminal window used to start Hadoop. We shall refer to this window
as to “Hadoop window” and we shall use it later on.
(2) Run MapReduce Applications
In this step, we process two MapReduce applications available in the Hadoop installation. Both
applications are included in the hadoop-mapreduce-examples-2.7.3.jar file in
$HADOOP_HOME/share/hadoop/mapreduce.
This first application is the computation of Pi by executing the following command (type 3 lines listed
blow in one line):
$HADOOP_HOME/bin/hadoop jar
$HADOOP_HOME/share/hadoop/mapreduce/hadoop-mapreduce-examples2.7.3.jar pi 10 20
Another application that uses a regular expression to search for all strings that start from a string
dfs. First create a folder input in HDFS in the following way.
$HADOOP_HOME/bin/hadoop fs -mkdir input
Next, process the following command to copy the files from etc/Hadoop at a local file system into
input folder at HDFS.
$HADOOP_HOME/bin/hadoop fs -put /etc/hadoop/conf/* input
Next process an application that uses a regular expression to search for all strings that start from a
string dfs.
$HADOOP_HOME/bin/hadoop jar
$HADOOP_HOME/share/hadoop/mapreduce/hadoop-mapreduce-examples2.7.3.jar grep input output ‘dfs[a-z.]+’
Note, that to perform the above operation, if the output folder exists in your HDFS, you need to
remove it first.
Check the outcome of this application through listing the contents of a folder output.
$HADOOP_HOME/bin/hadoop fs -cat output/part*
Finally, remove the folders input and output from HDFS.
$HADOOP_HOME/bin/hadoop fs -rm input/*
$HADOOP_HOME/bin/hadoop fs -rmdir input
$HADOOP_HOME/bin/hadoop fs -rm output/*
$HADOOP_HOME/bin/hadoop fs -rmdir output
(3) YARN user interface
Enter a link bigdata-virtualbox:8088 into your web browser. Check the list of applications
you just submitted and completed.
(4) How to compile WordCount.java Java program
Download a file WordCount.java and use editor (gedit) to browse its code. The functionality of
WordCount application and its implementation has been thoroughly discussed during the lecture
classes.
To set appropriate environment for compilation of a program process the following command.
export HADOOP_CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath)
To compile a program process the following command.
javac -classpath ${HADOOP_CLASSPATH} WordCount.java
Finally, we have to create jar file in the following way.
jar cf WordCount.jar WordCount*.class
(5) How to process WordCount.java Java program
Next, upload to HDFS home folder any text file. For example we can use a file WordCount.java,
please see below.
$HADOOP_HOME/bin/hadoop fs -put WordCount.java .
WordCount application will count the total number occurrences for each word in the file. Finally,
process WordCount application in the following way.
$HADOOP_HOME/bin/hadoop jar WordCount.jar WordCount WordCount.java /user/output/
Note, that an input parameter WordCount.java determines an input file located in HDFS in
/user/bigdata folder. An input parameter /user/output determines a location of the
output files in HDFS.
To see the results from processing of WordCount application process the following commands that
lists the contents of a file created in HDFS.
$HADOOP_HOME/bin/hadoop fs -ls /user/output
$HADOOP_HOME/bin/hadoop fs -cat /user/output/part-r-00000
The first command lists the contents of output folder created by Hadoop and the second command
lists the contents of a file with the results of counting.
(6) How to clean after processing of WordCount application
Before we can process WordCount application again we have to remove output folder created by
Hadoop. Of course, it is also possible to re-process WordCount application with a different value of
parameter that determines a location of the results.
To delete a folder output we have make it empty first with the following command.
$HADOOP_HOME/bin/hadoop fs -rm /user/output/*
Then, we can delete the folder with the following command.
$HADOOP_HOME/bin/hadoop fs -rmdir /user/output
And verify the results with
$HADOOP_HOME/bin/hadoop fs -ls /user
To remove WordCount.java from HDFS process the following command.
$HADOOP_HOME/bin/hadoop fs -rm WordCount.java
(7) How to process MinMax.java Java program
MinMax application reads the key-value pairs and for each distinct key it finds the largest value
associated with a key. Its functionality is the same as the following SQL statement.
SELECT key, MIN(value), MAX(value)
FROM Sequence-of-key-value-pairs
GROUP BY key;
Use an editor to examine the contents of a file MinMax.java. The application has a very similar
structure to WordCount application. The Mapper is almost identical. The Reduce instead of
summations find minimal and maximal values associated with each key.
To set appropriate environment for compilation of a program process the following command.
export HADOOP_CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath)
To compile a program process the following command.
javac -classpath ${HADOOP_CLASSPATH} MinMax.java
To create jar file process the following command.
jar cf MinMax.jar MinMax*.class
Next, upload to HDFS home folder a file sales.txt and list its contents in HDFS.
$HADOOP_HOME/bin/hadoop fs -put sales.txt .
$HADOOP_HOME/bin/hadoop fs -cat sales.txt
Before processing MinMax application, make sure that a folder /user/output does not exist in
HDFS.
$HADOOP_HOME/bin/hadoop fs -ls /user
If the folder exists then repeat step (2) above.
Finally, start MinMax application in the following way.
$HADOOP_HOME/bin/hadoop jar MinMax.jar MinMax sales.txt /user/output/
To see the results from processing of MinMax application process the following command.
$HADOOP_HOME/bin/hadoop fs -cat /user/output/part-r-00000
Repeat step (2) to remove the results of processing MinMax application.
(8) How to process Filter application
An objective of Filter application is to select from the contents of a file sales.txt all pairs such
that amount of sales is greater than a given value. Its functionality is the same as the following SQL
statement.
SELECT key, value
FROM Sequence-of-key-value-pairs
WHERE value > given-value;
Use an editor to examine the contents of a file Filter.java. The application has a simpler
structure than WordCount and MinMax applications. The Mapper is almost identical and there is no
Reducer. Please see the following line in a Driver main().
job.setNumReduceTasks(0); // Set number of reducers to zero
To set appropriate environment for compilation of a program process the following command.
export HADOOP_CLASSPATH=$($HADOOP_HOME/bin/hadoop classpath)
To compile a program process the following command.
javac -classpath ${HADOOP_CLASSPATH} Filter.java
To create jar file process the following command.
jar cf Filter.jar Filter*.class
A file sales.txt is already uploaded to HDFS, such that there is no need to do it again.
Before processing Filter application, make sure that a folder /user/output does not exist in
HDFS.
$HADOOP_HOME/bin/hadoop fs -ls /user
If the folder does exist then repeat step (2) above.
Next, we create a parameterized shell script to process Filter application with a parameter that
determines the minimal sales from which the transactions are listed. Start gedit editor and open an
empty file run in the following way.
gedit run.sh
Next, type into a file run.sh the following line.
$HADOOP_HOME/bin/hadoop jar Filter.jar Filter $1 sales.txt /user/output/
Note, that $1 represents a parameter of shell script run.sh. In the future we shall use the shell
script to process a line listed above and to replace $1 with a value of actual parameter. Save a file
run.sh and quit an editor.
Next, we have to grant processing (execute) privileges to a user on a shell script run.sh. To do so
process the following command.
chmod u+x run.sh
Finally, to run Filter application process the following command.
./run.sh 45
Note a value of actual parameter 45 at the end of a command line. The parameter means that we
would like to retrieve only sales whose value is greater than 45. When the shell script is processed a
value 45 replaces a formal parameter $1.
To see the results from processing of Filter application process the following command.
$HADOOP_HOME/bin/hadoop fs -cat /user/output/part-r-00000
Repeat step (2) to remove the results of processing Filter application.
(9) How to process Grep and InvIndx applications ?
Grep application finds the words that match a given template provided as a regular expression. You
can use command shell scripts c.sh, j.sh, load.sh, r.sh, show.sh, clean.sh in the
following way.
To compile the application, process a shell script c.sh in the following way.
./c.sh
To create jar file, process a shell script j.sh in the following way.
./j.sh
To load a file grep.txt, process a shell script load.sh in the following way.
./load.sh grep.txt
To run the application process shell script r.sh in the following way.
./r.sh grep.txt
To display the results process a shell script show.sh in the following way.
./show.sh
To remove the results process a shell script clean.sh in the following way.
./clean.sh
It is quite easy to adopt the shell scripts listed above to process an application InvInd that creates
an inverted index.
End of Exercise 2

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