DatabaseCache must implement incr to guarantee atomic increment

[Vinay Anantharaman]

django.core.cache.backends.db.DatabaseCache uses BasesCache's incr which does a get followed by a set. If the application uses multiple servers the increments will get clobbered. Furthermore, the current implementation overwrites the expiry time of a key to the default value instead of using the value which was initially set.

[Russell Keith-Magee]

You are correct that the implementation of incr on the DB cache backend will be subject to race conditions, so we should fix that.

However, I'd also say that if you're using the database cache backend for a count where there is heavy traffic, You're Doing It Wrong(TM). You should be using Redis, or at least Memcached.

[Vinay Anantharaman]

I am not doing it wrong necessarily because our DB is very small with a limited number of users. The table should basically live in InnoDB's buffer pool. It's not unreasonable to just use the DB as a cache for values.

[Michael Manfre]

My initial patch and pull request will not work as desired. Wrapping the get/set in an atomic block will only work for database isolation levels higher than Postgresql's default. There are two possible ways of ensuring an atomic incr, (1) control the locking from python, or (2) use select_for_update. Select_for_update would be the ideal solution, except DatabaseCache doesn't use the ORM and would need to expose an API for each database backend to provide SQL.

[Mike Lissner]

I may take a look at fixing this ancient but pretty small issue.

DatabaseCache doesn't use the ORM

Anybody know why this is? Seems like using the ORM would make life easier here, wouldn't it?

[Mariusz Felisiak]

Replying to Mike Lissner:

DatabaseCache doesn't use the ORM

Anybody know why this is? Seems like using the ORM would make life easier here, wouldn't it?

See #18401.