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Hadoop workflow

Local dev environment

See the Hadoop dev env notes for downloading and setting up the necessary libraries.

The steps below are required for every new Hadoop application you create.

Eclipse

Create a new Java project, as usual.

Eclipse 1

In the “Libraries” tab of the new project window, click “Add External JARs…”

Eclipse 2

Find the jars folder in the extracted hadooplibs ZIP file. Add every JAR here.

Eclipse 3

Finish creating the project as usual.

Next, you need to create a Java source file. Right-click on the project name, “WordCountEclipse”, and select New > Class.

Eclipse 4

Name the class WordCount (just for this example). Write some code. See the MapReduce demo for the WordCount class source as an example.

Eclipse 8

Next, you need to create a “run configuration” that tells Eclipse how to execute your code. You already have a main() function in your main class, so we can run the code as a typical Java application. However, you do need to specify the input and output folders since the WordCount example expects these as inputs to the main() function.

Right-click on the project name, “WordCountEclipse”, and select Run As > Run Configurations. Create a new run configuration modeled after the “Java Application” template.

Eclipse 8

Specify the class with the main() function. Also name your run configuration.

Eclipse 6

Click the “Arguments” tab and add the names of the input/output folders as program arguments.

Eclipse 7

Then close the run configurations window.

Finally, create a folder called input (or whatever you named it in the run configuration program parameters), and add some text files in there. The WordCount example will read these text files and count the words. Below is a screenshot of some example files.

Eclipse 8

Now you can run your program. Click the Run button (green/white arrow). If successful, your program will create a directory called output (or whatever you named it in the run configuration program parameters), and output a file called part-r-00000 with word counts. You should also see log messages in the console window at the bottom of Eclipse.

Note: Your program will crash if the output folder already exists. You must delete it before every run. If using HDFS, you can use a command like the following (change the username!):

hdfs dfs -rm -r /users/jeckroth/wordcount/output

Eclipse 10

You can also debug your program, in the usual way. Here is an example of a breakpoint in the map() function. Clicking the Debug button (left of the normal Run button) will open a debug interface where you can inspect function arguments, etc.

Eclipse debugging

Notice, in this example, that the argument value to the map() function is the first line of text from the Iceland Wikipedia page. This text comes from a file in the input folder. The key variable has the value 0 (not shown in the screenshot).

Eclipse debugging

delenn environment

First, put some files into HDFS:

hdfs dfs -mkdir -p /users/jeckroth/wordcount/input
hdfs dfs -put input/* /users/jeckroth/wordcount/input

Or, you can use existing data, such as: /data/westburylab-usenet/WestburyLab.NonRedundant.UsenetCorpus.txt

Copy your JAR file to delenn. If you get an error about the Java version, you’ll need to compile your code on delenn.

How to compile on delenn

Copy your source files, and compile them as follows:

javac -cp `hadoop classpath`:. *.java

Then make a JAR (name your JAR whatever you want; wc.jar is used here), naming the main class as the third argument:

jar cfve wc.jar WordCount *.class

Submit the job

Submit the job as follows:

yarn jar wc.jar /users/jeckroth/wordcount/input /users/jeckroth/wordcount/output

In the command above, WordCount is the class with the main() function, and the rest (the file paths) are just arguments to main(). Your main() may have different kinds of arguments, or no arguments.

Monitor your job’s status on the ResourceManager’s web interface.

When it’s done, get the output out of HDFS:

hdfs dfs -get /users/jeckroth/wordcount/output/part-r-00000

…or use the web interface.

londo environment

Since londo is not really running a cluster (virtual or otherwise), you will not use HDFS on londo. Just run YARN directly, after transferring your JAR or compiling it on londo:

yarn jar wc.jar my-input.txt output-folder
CINF 401 material by Joshua Eckroth is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Source code for this website available at GitHub.