If we externalize our runtime database with PostgreSQL and utilize PostgreSQL replication to ensure we have backups of each instance, is there still a need to backup the Dataiku DATADIR for disaster recovery purposes?
You MUST backup the DATADIR directory. The runtime database only contains runtime data. You can wipe your runtime database and still have all your projects data. In an ideal situation you should shutdown DSS and perform a DATADIR directory backup and a runtime database at the same time to have a consistent backup between the two. But like like I said you can survive without the runtime database but not without the DATADIR directory.
