https://www.connexitor.com/blog/pivot/entry.php?id=105#body https://www.openldap.org/lists/openldap-software/200702/msg00006.html https://www.openldap.org/lists/openldap-software/200602/msg00064.html
MirrorMode is a hybrid configuration that provides all of the consistency guarantees of single-master replication, while also providing the high availability of multi-master. In MirrorMode two masters are set up to replicate from each other (as a multi-master configuration) but an external frontend is employed to direct all writes to only one of the two servers. The second master will only be used for writes if the first master crashes, at which point the frontend will switch to directing all writes to the second master. When a crashed master is repaired and restarted it will automatically catch up to any changes on the running master and resync.
This is discussed in full in the MirrorMode section below