Another long-needed update has been made to the Open Directory replication scheme.
In previous releases, Open Directory replication operated on a hub spoke system with a single Open Directory master and a single tier of replicas.
Though effective, this single-tier approach limited how scalable replica sets could be and how flexible replication design could be in varying network topologies (particularly in multisite topologies).
Open Directory 4 supports continues to supports a single Open Directory master, but that master is now limited to 32 replicas, each of which can also support 32 replicas—allowing for better replication performance as well as decreased load on the Open Directory master (and therefore better overall performance).
This two-tiered system is much more flexible and scalable. Managing replication, however, remains relatively simple and, with the exception of the ability to design a replication tree, maintains a very similar interface to previous versions.
The limitation to 32 replicas of each master, however, means that environments that currently have a single master with more than 32 replicas will need to be redesigned.