The edX modulestore stores the course content for all versions of all courses and is backed by MongoDB.

http://edx.readthedocs.io/projects/edx-developer-guide/en/latest/modulestores/index.html

Notes From Modulestore Informational Meeting

https://docs.google.com/a/edx.org/document/d/1OEXLvInQt6Wrr5w3JxLbAHJghF5P8RuFVq2oW-CfLgk/edit?usp=sharing_eid

Mixed Modulestore (Facade)

https://github.com/edx/edx-platform/wiki/Mixed-Modulestore

Different Modulestore Iterations

XML

Old Mongo

Split

MongoDB and the Modulestore

Why/how does the modulestore use MongoDB? This presentation might help:

http://doctoryes.github.io/mug_talk_modulestore/#1