Apache JCR with failover - Hadoop?
The JCR 2.0 RI Jackrabitt "Oak" provide a MongoDB integration (MongoDB Storage) that supports all the features of MongoDB (Replication & Sharding)
You cand find information here:- http://jackrabbit.apache.org/oak/docs/dev_getting_started.html
I have just recently Open Sourced a JCR Browser that is built on Apache Oak JCR + MongoDB + SpringBoot and has a front end on JQueryMobile. It is on GitHub (including all source), and here is the site too: http://meta64.com