Caching behavior

[henadzit]

At the moment django-dynamic-preferences caches values indefinitely if caching is used. There are potentials issues with this approach where the database and the cache get out of sync. There are a few examples of how this could happen:

• The process crashes or getting killed just after the setting has been stored in the database but the cache hasn't been updated yet
• A database backup is applied with an older version of the setting
• A redis backup is applied when redis is used as a cache backend

Having a timeout would also let to update the setting in the database directly without using Django. At the moment this would require clearing cache manually.

I see three potential paths to introduce more flexible caching:

• Remove None from arguments of the call self.cache.set(key, value, None) here. This will make Django to use the default cache timeout.
• Add a global option to specify the timeout. This approach seems to be used in similar libraries.
• Add a configuration option for timeout per preference.

Please let me know what you think and I can help with PR for this change.

[agateblue]

Hi there, I think it make sense to at least remove the None param in the cache.set to use the default cache value. Having a dedicated setting dynamic_preferences for this (defaulting to the default cache timeout) would also work great!

[henadzit]

@agateblue I've created PR for using the default cache timeout. Could you please have a look?