Being a graph database user, i always have to manage a replication of the "molecules"of my graph in ES for a user-friendly search experience. Ca arangodb help for such a use case? Or may be dgraph?

With ArangoDB you can choose between synchronous replication and asynchronous. With the Agency of ArangoDB you also have a RAFT based consensus protocol which holds the state of the cluster. My team mate wrote a nice article about our approach. You might want to have a look: https://www.arangodb.com/2017/01/reaching-harnessing-consens.... In single instance you have full transactional guarantees with multi collection and multi document transactions. In cluster mode we provide single document transactions. More guarantees will follow.

Dgraph does automatic replication, for providing fault tolerance. It's baked in pretty deep into Dgraph. We use consensus algorithm, so all your writes are atomically consistent (not eventually consistent). https://docs.dgraph.io/v0.7.4/deploy/