I recently heard about RethinkDB on a few podcasts (SE Radio and The Changelog) and it sounds very promising, and has been designed to solve real time web application use cases very well. Glad to see they now support Java so we can use it easier from JVM based languages
I would also like to point out that it's a pretty good document store even without the super awesome realtime bits:
This video is 2 years old (and rethinkdb has gotten way more awesome since then), but it comes with some super useful things out of the box, like a very nice Web UI, distributed system support, joins, and a great way to write complex queries (ReQL):
https://www.youtube.com/watch?v=qKPKsBNw604
If a document model fits your data, I'd suggest giving rethink a try.
