def replicate(self, doc_id, source, target, continuous=False):
        """
        Starts a replication from the `source` database to the `target`
        database by writing a document with the id `doc_id` to the "_relicator"
        database
        """
        if doc_id in self["_replicator"]:
            return
        data = {
            "_id": doc_id,
            "source": source,
            "target": target,
            "continuous": continuous
        }
        self["_replicator"][doc_id] = data