This is an old topic 2014-07-12 maybe this is not their stack now.
These are some concepts and techniques I have learned through OpenEdx.
Mysql: store users, students, courses, certificates and courseware progress. text saved as UTF-8, and time is saved ad UTC. It uses InnoDb engine In development environment, It uses sqlite instead of mysql to keep dependencies as low as possible.
Mongodb: store courseware contents, discussion data, comments
Event Tracker: logs the events as JSON and saved them in log files. Event example: student events.
Django and some dependencies:
XBlock: is a component architecture, to build small block to large one. It can be extended and customised.
XQueue: grading service
ORA: collect grades from queue , machine learning grading and staff grading.
CS Comments Service: is an independent comment system which supports voting and nested comments.
CodeJail manages execution of untrusted code in secure sandboxes.
notifier: send messages by Email or SMS to subscriber.
Discussion forum: uses Elasticsearch, Forum and ruby