I also first used MongoDB but I switched to cassandra and I find it much better. MongoDB was bad for this case because it devours memory and it lacks a primary

I also first used MongoDB but I switched to cassandra and I find it much better. MongoDB was bad for this case because it devours memory and it lacks a primary key (so saving a tick that's already creates a second tick). cassandra is better (it has a PK, although it's a bit weird because of its distributed-first attitude) but in the long run I think postgres would be even better (because it's space efficient). Apart from that I use some Java libraries and clojure/incanter and program the rest around it myself.