Clustering is one of the most common unsupervised Machine Learning tasks. Solr is shipped with a clustering module based on Carrot2 built-in algorithms. Carrot2 comes with 4 algorithms: Lingo, STC, kMeans and Lingo3D each one mapped to a clustering engine. The first three are open-source whereas the last one is commercial. When this approach is used, clustering takes place in memory. Other frameworks, such as Mahout, can be used to do the clustering “off-line.”