[Epic] MongoDB deprecation for forum

[salman2013]

Description:
After deprecating or removing 'cs_comments_service' (v1) usage, we are using forum (v2). Forum (v2) uses two backends for data storage: MongoDB and MySQL.

As per my understanding there are three things which need to be done under this ticket.

• Add MySQL support in edx-platform (Ref # 1, Ref # 2)
• Remove Mongodb backend usage in edx-platform
• Remove Mongodb backend from forum (Ref)

References:
Forum v2 repository: https://github.com/openedx/forum
Tutor forum plugin: https://github.com/overhangio/tutor-forum
Legacy forum (deprecated): https://github.com/openedx/cs_comments_service

Parent Ticket:
#424

As per the parent ticket, the following scenarios were tested:

• Integration of edx-platform with forum v2:
  • With mysql backend
  • With mongodb backend
  • With both mongodb+mysql backends for different courses
  • With Elasticsearch as a search backend
  • With Meilisearch as a search backend
• Compatibility of forum v1 in edx-platform:
  • By using forum v1 exclusively
  • By using forum v2 exclusively
  • By using forum v1 and v2 for different courses
• Data migration from mongodb to mysql: making sure that data is preserved and compatible with forum v2.

[feanil]

The work that is left on the parent DEPR is to drop mongo support from the forums code. I believe this comment confirms that. I believe the outstanding questions are:

• Are there waffle flags/switches that need to be removed?
• What code in the openedx-platform needs to be removed?
• Is there code in the forum library that can be dropped?

There may be other questions but we need to answer at-least these before proceeding with the work.

[taimoor-ahmed-1]

The work that is left on the parent DEPR is to drop mongo support from the forums code. I believe this comment confirms that. I believe the outstanding questions are:

• Are there waffle flags/switches that need to be removed?
• What code in the openedx-platform needs to be removed?
• Is there code in the forum library that can be dropped?

There may be other questions but we need to answer at-least these before proceeding with the work.

• Are there waffle flags/switches that need to be removed?
  forum_v2.enable_mysql_backend (forum/toggles.py:9-18) A CourseWaffleFlag that switches between MongoDB and MySQL backends. When MongoDB is dropped, the flag and its associated logic in forum/backend.py:9-30 can be removed entirely.

• What code in the openedx-platform needs to be removed?
  None the v1 and v2 support toggles were removed earlier.

• Is there code in the forum library that can be dropped?
  Yes absolutely,

forum/mongo.py
forum/management/commands/forum_create_mongodb_indexes.py
forum/management/commands/forum_delete_course_from_mongodb.py
tests/test_backends/test_mongodb/ ...(and more)

Dependencies to remove from requirements if we opt to remove the management command for data migration from mongo-> mysql. I would suggest to not remove that script immediately as people would need it to move their existing data on mysql.

pymongo
mongomock (test dependency)

[taimoor-ahmed-1]

@salman2013 here is the regression sheet we used for testing the discussions platform

discussions_mfe_testing_documentation.xlsx