tuts and config examples:
https://blog.karmacomputing.co.uk/how-to-create-mongodb-replica-set-with-authentication-ubuntu-16-04/ https://www.digitalocean.com/community/tutorials/how-to-implement-replication-sets-in-mongodb-on-an-ubuntu-vps http://blog.brunobonacci.com/2013/02/22/how-to-convert-a-standalone-mongodb-into-a-replica-set/ https://docs.pritunl.com/docs/securing-mongodb https://medium.com/@rossbulat/deploy-a-3-node-mongodb-3-6-replica-set-with-x-509-authentication-self-signed-certificates-d539fda94db4 https://medium.com/ninjaconcept/how-to-quickly-setup-mongodb-on-digitalocean-3d9791a7aaa4 https://gist.github.com/davisford/bb37079900888c44d2bbcb2c52a5d6e8