After the connectors' state has been persisted, little maintenance is done on the failover installation. idsync resync -u should be run periodically as described in Periodic idsync resync Operation for Failover Installation.
If any additional attributes are added to the list of synchronized attributes, a full idsync resync -u operation should be executed, not an incremental one. If this is not done, when the Active Directory Connector is started after a failover, it will incorrectly detect modifications to the added attribute because its object cache database did not store the previous value.