afex

or the moment you have more than one application running. (which for many products is day #1)

i go one step further, and put the domain layer behind a network interface (http or other) with a versioned API. IMO, trapping behavior inside a single application is just as bad as tying it to a framework. the domain layer needs to be available to the entire organization.

rails is just a tool to build a front-end app. i have many of those on my project. some rails, some sinatra. if they want to access shared behavior or data, they include the appropriate gem which knows how to access a particular API.

As far as datastores go, cassandra is write-optimized. Writes are faster than reads. So for this use case (heavy denormalization) it is a good fit.

Also, Cassandra reduces the operational complexity of having a logical store which spans multiple hosts. Your memcache example does not get persistence for free, and sharding mysql is something you have to do manually. The interface to a Cassandra cluster is the same regardless of how many nodes you are running.

Scaling writes is "hard". Incrementing counters is obviously write heavy, and cassandra aims to make it easier.