Question

We are using octopus in our rails app to forward read queries directly to our slave boxes, and writes to our master. Have to say its a great gem, but we noticed that queries to the slaves forgoes Active Record’s default SQL caching. Kind of defeats the purpose to scale the DB servers horizontally only to lose out on the caching layer that would help scale.

Does anyone have an idea on how to fix this, or is there a better gem to use. We don’t need the sharding functionality that octopus gives; just the replication.

thanks ahead of time

Answer 1

The way SQL caching is turned on for a connection is by doing something like

ActiveRecord::Base.connection.cache do
    # the query cache is used here
end
# and turned off by here

For the main activerecord connection rails has a rack middleware that sets this up for each request. If you want this to happen for your slave connection then you’ll need to do something similar yourself to enable caching for your second connection (slightly easier than a rack middleware would be an around_filter)

I’m not very familiar with octopus, but it seems likely that these two connections will be independant – activerecord won’t know to invalidate its cache on your read connection just because you’ve done some writes on your other connection