[prohor]

The license is Affero GPL, which means you need to open-source your code even if you use it for a service. "Traditional" GPL was effective only while redistributing. That means you would need to go for commercial license whenever you build a service on it. Which in fact is a fair approach for a business model when there is a company behind an open source project. Especially that this time there is no lock-in. You could always come back to Cassandra.

[mappu]

The virality doesn't cross the database interface layer.

Modifications to the database software must be shared, yes, but your client application is outside the reach of the AGPL and can remain proprietary.