It was one of those, "we need to do something now and this will work" solutions. We had a really talented consultant working for us, writing some of the early code. He was familiar with Mongo and wanted to go that route. Early on I said we should use Cassandra for this, but it took us quite some time to get to the point of being able to migrate. A testament to his code and the "this will be temporary" foreknowledge is the fact that we could swap data stores in a pretty massive sense without much in the way of outward-facing changes to our processing engine.

I think the hard thing for us up front was that trying to explain Cassandra data models to someone (and this guy is really really good) and then hand the rest of the work over to them to implement now, on a contracting rate is not a trivial problem. And we needed to hurry for both deadlines and burn rate.