[jmalicki]

If you're not just running it all on one db, why do you have to have all of your different DBs be on the same DB software?

I used to work at Yelp, and at least at that time, a big use case of Cassandra was basically for what were essentially materialized views created from log data. At that point in time, MySQL or logs were the "sources of truth", but there were enough transactions going on that it made sense to have things like Cassandra around too for some of the other use cases.