Generally DL does not know the structures of the message. Currently when it insert messages into Elasticsearch, it let ES to generate a random key/id. For Couchbase, it retrieves a topic specified sequence id from Couchbase, and the DB key is made up as topic_name+this sequence . 

For some cases, it may be useful to allow us to specify how to construct the key using some JSON field for messages in a particular topic. Admin should be able to do this through admin UI.