Mahout !

Mahout is an incredible library to do machine learning, clustering, classification, recommendation. It works directly on top of Hadoop and MapReduce !

This is how to launch a recommender Job :

hadoop jar /usr/lib/mahout/mahout-core-0.7.0.21-job.jar org.apache.mahout.cf.taste.hadoop.item.RecommenderJob --input /apps/hive/warehouse/profile_activity_text_file --output /apps/hive/warehouse/recommenderJob --similarityClassname SIMILARITY_COOCCURRENCE --booleanData true