Notifications do not show up in the user interface

[markkuriekkinen]

After A+ v1.8, notifications have stopped working at least in the production A+ server. In local testing, the notifications are still working. This could imply that the issue is caused by the cache or cache configuration.

I have tested and verified that the notification rows are added to the notifications table in the database (in production). The seen attribute is false.

It is worth noting that A+ never deletes old notifications from the database. The production database contains over 50000 unnecessary old notifications. They could be removed.

Teacher's message:

We noticed that we're not (reliably) getting the normal notifications for received messages in the top menu of A+. We've tested with reviews from Rubyric, replies to feedback messages from MOOC-Jutut and (manual assistant) reviews to exercise submissions on A+.

Steps to reproduce:

• Submit feedback with an end of chapter form, or make a regular exercise submission.
• (As course staff) Answer the feedback message in MOOC-Jutut, or review the submission in A+ (assistant feedback).
• (Back on the original user) Refresh course page.
• Red notification in the top menu (top nav bar) should appear, but doesn't.
• Exercise results page does show that there is feedback available for the submission.

One time I 'reviewed' a submission on A+, the notification and two earlier ones did appear for my student account, but I have not seen any since then for a few tests.

Internal ticket: https://rt.cs.aalto.fi/Ticket/Display.html?id=18169

[markkuriekkinen]

Relevant code:

• https://github.com/apluslms/a-plus/blob/ae4aef150899ad3be6402fd37c90345b6713d8c2/notification/templatetags/notification.py#L12

• https://github.com/apluslms/a-plus/blob/ae4aef150899ad3be6402fd37c90345b6713d8c2/notification/cache.py#L7

• https://github.com/apluslms/a-plus/blob/ae4aef150899ad3be6402fd37c90345b6713d8c2/lib/cache/cached.py#L10

• https://github.com/apluslms/a-plus/blob/49b229a222574e73411bd6929efb6b519870e39a/exercise/async_views.py#L60

• https://github.com/apluslms/a-plus/blob/ae4aef150899ad3be6402fd37c90345b6713d8c2/notification/models.py#L35

[markkuriekkinen]

It is curious that sometimes the notification shows up in the UI like it should, but not every time. There are many recent notifications that have been shown to the user, but there are also many that have not been shown.

[markkuriekkinen]

New admin CLI command for deleting old notifications from the database: https://github.com/apluslms/a-plus/commit/fbed17c2ffcf6bd46a005264a50fdbe9bf961518

[markkuriekkinen]

I think that the code itself works OK, but if there are too many notifications, the cache stops working. The user interface can not show the notifications if they can not be read from the cache. Deleting old, unused notifications from the database alleviates the situation. It is still an open question what could be improved in the notifications cache so that it would work more reliably.

It is hard to specify what exactly goes wrong with the cache, but at least the cache has limited size for the whole cache and the individual item size. It might silently drop items that are too large.

[markkuriekkinen]

Django 3.2 LTS replaces the old Python Memcached API with a new library. I don't know if the new Memcached API/library could fix some problems in the cache.

[markkuriekkinen]

Closing. I assume that everything works if we regularly delete outdated notifications from the database so that the number of notifications does not increase all the time.