To use ZDT, you have two environments: a passive environment and an active environment. You use Oracle GoldenGate to manage the data replication and message streams between the environments. While you upgrade the passive environment, MDM users can access the MDM Hub in the active environment. Any change made to the MDM Hub during the upgrade does not impact the ability to replicate changes from the active environment. Backfill mechanisms handle any impact to the MDM Hub metadata caused by the upgrade.