Have you looked at couchdb? It has an http api and you can "watch" the modifications made to the database using a "_changes" feed that has options for longpol

Have you looked at couchdb? It has an http api and you can "watch" the modifications made to the database using a "_changes" feed that has options for longpoll, eventsource and a "continuous poll". Web socket is not supported out of the box yet but you can always put couchdb in front of a proxy and send the changes feed over the socket connection once it's established.