https://docs.google.com/presentation/d/1u4EoOPwrsQPdZR5Jp9ny1EnRpPgUwsTIybuydkkt4xg/edit?pli=1#slide=id.p
http://edx.readthedocs.org/projects/edx-developer-guide/en/latest/modulestores/split-mongo.html#split-mongo-modulestore
Paul-Olivier and Xavier
https://groups.google.com/forum/#!topic/edx-code/73jNj2BMU04