Why are you choosing Cassandra every time? And what's your refutation for his desire to use MySQL over some other database, from his point of view as presented in the article (e.g. availability of support via Google/SO)?
Linear horizontal scaling (double capacity? double # of nodes), no SPOF, Active/Active/Active cross-WAN HA.
> And what's your refutation for his desire to use MySQL over some other database, from his point of view as presented in the article (e.g. availability of support via Google/SO)?
Hire people that can learn DB internals and be the expert you want to see in the world: http://stackoverflow.com/search?q=user:4774369+[cassandra]
Cassandra and similar technologies constrain your queries and make schema changes incredibly expensive. This is the opposite of what you need in a startup. You need flexibility.