MapReduce jobs to find the connected components in a large graph
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What is this? Is it is awesome?

Componentize is an implementation of the connected component MapReduce algorithm found in Graph Twiddling in a MapReduce World.

It is my understanding that it is pretty cool. I don't know about "awesome", yet. We'll see.


(The why and wherefore: Componentize is written in Scala, uses Hadoop 0.20.1 to distribute the work, and Buildr for building the source code.)

Doing the Thing

Building the Jarfile

To run this pretty little creature, you'll need to create it's jar file.

To do that, you'll run buildr package (or jbuildr package if you are on OS X and have to use the JRuby version). However! However, you will need in your CLASSPATH the Hadoop 0.20.1 core jar, all of the Hadoop's depedencies (found in the lib directory of the 0.20.1 tarball, natch), and, of course, the Scala jarfiles.

A couple of nice things:

  1. Buildr will go out of its way to find the Scala jarfiles and add them for you. I know for a fact, in fact, that if you install Scala through MacPorts on OS X, the jbuildr command will find your Scala jarfiles and add them for you to the classpath Componentize will be built with.

  2. If you create a lib directory in your checkout of Componentize and copy all of the Hadoop and Hadoop dependency jarfiles to it, you will magically have them added to classpath used to build Componentize.

In any case, the jarfile will appear in the target directory as componentize-$someversion.jar.

Running the Hadoop job

The jarfile created in the target directory by buildr package can then be added to your HADOOP_CLASSPATH, or just run with the usual single-core, single-machine Hadoop command like so:

$HADOOP_HOME/bin/hadoop jar target/componentize-$someversion.jar \
  componentize.Main -edgedir edges/


Before you do that, you'll need some input data! In the edgedir I've slyly mentioned in the above command, you'll need one or more files with data formatted as:

 nodeid1	nodeid2
 nodeid56	nodeid1000

and so on. Let's be clear here, those are big fat TAB characters in the middle there. TAB as in "\t"; as in that key over "Caps Lock". They are not spaces.

You can find some example input data in the examples directory.


The output of Componentize is ... verbose. And I don't just mean what's printed to /dev/stdout. In the output directory, you'll have 3 directories of output for each iteration of Componentize plus one for the set up. This is not a lot of fun.

However, when Componentize finishes it will be a good sport and log what directory you can find your output. This directory will be a directory starting with zonefiles and ending in a number. It will also be the zonefiles directory with the greatest number at the end.

This output directory will be filled with one or more tab and newline delimited files where the first column is the component label (or "zone") of the node in the second. As in:

zone1	nodeid73
zone2	nodeid100

Things That Would Be Nice

  • We should use Combiners to reduce our load.

  • There is probably a better way of storing the transient data between jobs than just simply writing out another output directory with SequenceFiles in it.

  • There is no way to do only some of the processing. It is all of the set up and phases or none at all. Though, we could just make it easier for someone to configure the Phases themselves.


On build, we use Scala X-Ray to build a nice HTML version of the code. Check it out in target/classes.sxr/index.html after you compile Componentize.