: Initially, I was looking at http://wiki.apache.org/solr/MoreLikeThis : : Then, it turned out that most implementations are based on a combination of : Mahout, Solr and Hadoop.
I think you'll find that most "serious" (for some definition) content recomendation engines use various ML algorithms (ie: mahout) to crucnh both the content and the (aggregate) user behavior data to generate "people who like this thing also like..." and "people like you also tend to like..." type recomendations. But Solr, with and w/o MLT, can be very handy for "things similar to this thing are..." type depending on how you use it. (I don't think i'm allowed to name names, but i can think of a couple of major www sites of which i have first hand knowledge that use MLT and/or customized things like MLT to serach their Solr/Lucene indexes for "things similar to this thing you are currently looking at"). -Hoss
