Clustering_with_Hadoop

Introduction

MapReduce is a programming model that assists in dealing with large datasets. Useful across many domains, one specific application for which the MapReduce paradigm has been explored is knowledge discovery from nuclear reactor simulation data, as it is imminent that this data will quickly become large-scale. Hadoop, an open-source implementation of MapReduce, was used for this study.

For preliminary investigation, k-Means clustering, available from Mahout was employed. The results were similar to the ones we published in the paper, "Knowledge Discovery from Nuclear Reactor Simulation Data".

Process

Here are the steps we used for working with Hadoop:

• Successfully install Hadoop from their Releases page. I used a tutorial by Michael Noll to install Hadoop on my laptop, running on Fedora 17, as a single node cluster. (Note that this tutorial is only helpful for certain Linux users.)
• The database used was Hadoop Distributed File System. A helpful tutorial on HDFS can be found here.
• Successfully install Mahout, a scalable machine learning and data mining library, from their Downloads page. Examples on how to run various machine learning algorithms on Mahout can be found in /mahout/examples/bin.
• Put the requisite data for analysis on HDFS.

My data files are in the .csv format. Mahout's k-Means clustering expects the data to be formatted into a Mahout-specific SequenceFile format. A Java utility was written to convert our data into a dense sequence file format, as needed by the k-Means clustering algorithm. Please note that the examples for k-Means in Mahout mostly work with converting text data into sparse vector format. However, this was not the case with our data.

• Run k-Means with the -cl option. For example:

  mahout kmeans -i /data/diff_unc_trans_seq/part-m-00000 -o /data/diff_unc_trans_kmeans_op -x 20 -k 2 -c /data/analysis_clusters -cl

Information on the various command line options for k-Means can be found at http://mahout.apache.org/users/cluste.../k-means-commandline.html.

• Get the clusteredPoints directory out of HDFS into your local directory.

• Run mahout seqdumper to output them. For example:

  mahout seqdumper -i /data/diff_unc_trans_kmeans_op/clusteredPoints/part-m-00000 -o ~/Data_orig/diff_unc_trans_hadoop_op/.

This can be further processed for analysis.

Resources

For more information on Hadoop, see their website, which contains many useful resources, including a tutorial on how to use Hadoop.

More details on Mahout and its uses can be found at the Apache Mahout website. The developers keep the website very well maintained, and you can find a great introduction to k-Means clustering, in addition to information on many other methods.

Some further helpful materials on MapReduce include:

• Chapter 2 of Mining of Massive Datasets by Anand Rajaraman and Jeffrey David Ullman.
• MapReduce: A Flexible Data Processing Tool, a seminal paper by Google Fellows Jeffrey Dean and Sanjay Ghemawat, published in 2010.