Implications with south migration

[tsailiming]

Hi,

Do I need to restart memcache after I perform a database migration using South?

What happens to stale cache entries that may not reflect the latest changes in the model?

[jself]

Yes, it's a good idea to because columns may still be missing/etc, or any migrations that have happened won't appear in the cached version until a table write.

[tsailiming]

Also, I don't think cache versioning is supported yet? This is to avoid having to start memcache from the cold state?

Any suggestions how this can be implemented together with South to achieve the desired effect? Maybe like this:

1. By keeping an internal version column and South migration will increment it whenever the model's columns are added/removed/renamed.
2. Johnny cache to support versioning by looking at the version column and use it for cache versioning?

[jself]

No, since we invalidate the entire table on any write (hence you essentially get a cold cache every table write), there's not much point since previous generations don't need to be kept (and roll off naturally of memcache). Versioning would just look like general invalidation. Perhaps a feature would be to invalidate all tables, or certain tables based on a migration, but the cache isn't selective enough for versioning to give much of a preformance boost.

[tsailiming]

but the cache isn't selective enough for versioning to give much of a preformance boost.

Because of Johnny implementation?

My concern is that in my use case, model modifications are common but typically is limited to a few columns/tables at a time, especially when the product is not matured yet and due to the nature of CI development. Things change all the time but at a small delta each time.

Once I restart memcache, the initial loading will all be from the database even for those tables that are not changed when they are relatively static but commonly accessed such as country/city names. Though I could also warm up the cache by myself via a management command (but that will depend on the amount of cache that needs to be restored. Could be heavy on some sites that have GB of caches).

I'm guessing when the old model is deserialised and when I try to access the old/new properties, something will go wrong?

Would love to hear more opinions on how best to optimise this further.