RethinkDB is the latest major entrant NoSQL database. Having used it in production to build my current startup, it is in my opinion one of the best databases for a reactive web app. The key feature that makes it so powerful are changefeeds. These are essentially defined ReQL queries tied to an event listener (callbacks). Every time a document is changed, added, or removed that is selected by the query, the changefeed passes the updated document(s) to the event listener. If you're anything like me, I'm sure your mind is already spinning with the possibilities!