I have built applications that uses couchbase both on server side and couchbase-lite on Android. Couchbase with the ability to sync between client and server replication is the best thing to happen and available free (community edition) today.
Unfortunately couchbase-server consumes lot of resource on the backend and it's heavy, which brings an entry of barrier from a start up point of view.
https://github.com/asciimoo/searx