Apache NiFi Registry System Administrator's Guide
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Configuration Files

If using NiFi Registry's policy based authorization, the users, groups, and policies are stored in files on disk named users.xml and authorizations.xml. These files should be periodically backed up to an external location. In order to ensure a proper backup, NiFi Registry should be stopped to ensure no authorization data is being written to disk.

If using Ranger, then all authorization information is stored externally and there is nothing to back up.