We have found that for many courses on edx.org, the "current version"
of the course that's stored in MongoDB doesn't match what's stored in
MySQL. Although this doesn't directly cause any problems as the
"current version" (course index) is not normally read from MongoDB,
it's not supposed to happen - and it can cause problems if some other
extra-platform components (like a pruner script) read the course
indexes out of MongoDB.
We still aren't sure what can cause MongoDB and MySQL to get out of
sync in the first place; this won't necessarily fix that issue. What
this does fix is a bug that seems to be: once they get out of sync,
they stay out of sync and mongo will stop receiving writes.
The Mongo code in most cases will only write a new record if the
"current" record's last_update matches the last_update value before
the change was made. e.g. last_update is "10am", user makes a change,
Mongo gets updated only if the current row's last_update is still
"10am". Otherwise it's considered a "collision" and silently ignored.
Once Mongo and MySQL somehow become out of sync, they may stay out of
sync because any new writes will have a last_update value read from
MySQL, which is newer than the value in MongoDB, so the MongoDB writes
will all be rejected as "collisions".
This should fix the issue by making mongo writes always match the MySQL
writes, instead of letting the Mongo code "decide on its own" when to
write course index updates or not.
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