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CachedPoints may grow too large for Memcached and crash the course for those users #979

Open markkuriekkinen opened 2 years ago

markkuriekkinen commented 2 years ago

Memcached has a default max object size of 1 MB. It is possible to increase it, but it is not recommended. https://github.com/memcached/memcached/wiki/ReleaseNotes142#configurable-maximum-item-size

By making thousands of submissions in a course, students are able to reach the max object size for the points cache (CachedPoints) by making unusually many submissions. Since most A+ course pages use the points cache, if generating the user's points cache fails due to the object size limitation, then the user is never able to access the course pages because they always crash. A+ is not currently able to render course pages for a user whose points cache generation crashes to the object max size limitation.

https://github.com/apluslms/a-plus/blob/b43cdb4667aae989bde42a45cbd5b3251f80ef94/exercise/cache/points.py#L47

In minus.cs, one teacher reached the Memcached limit when he had made 6024 submissions in one course instance. At that point, opening, for example, exercise pages, course front page, or the teacher's participant overview all crash to the MemcacheServerError.

MemcacheServerError at /cs-a1120/2022/efficiency/efficiency-roots
b'object too large for cache'

A related internal Aplusguru ticket: https://rt.cs.aalto.fi/Ticket/Display.html?id=20537

Ideas for solutions

markkuriekkinen commented 2 years ago

Related:

PasiSa commented 2 years ago

excluding submissions in loaded case sounds a proper approach to me. Increasing cache size, or compressing it, helps in short term, but just postpones problems until someone creates even more submissions

markkuriekkinen commented 1 year ago

Did #1231 fix this? It significantly changed the cache functionality.