To allow Active Directory to detect changes that occurred while it was stopped, the Active Directory Connector persistently stores the usnChanged value of the last change that it processed. When Active Directory begins processing changes again, it only needs to examine entries with an usnChanged value larger than the previous one that it has processed. Like the Directory Server Connector, the Active Directory Connector first stores this usnChanged high-water mark when synchronization is started for the first time. Therefore, as part of preparing the failover installation, synchronization is started to allow the Active Directory Connector to check-point its state.
When failing over and synchronization is restarted for the failover installation, the Active Directory Connector will process all entries that were modified since it was last started. Even though this will be thousands of entries, the processing will be fast because the Active Directory object cache database is up-to-date with most of the changes. Only the changes made since the last time that the idsync resync command was run will be processed.