The UID uniqueness plug-in does not perform any checking on attribute values when an update is performed as part of a replication operation. This does not affect single-master replication, but the plug-in cannot automatically enforce attribute uniqueness for multimaster replication.
Because all modifications by client applications are performed on the master replica, the UID uniqueness plug-in should be enabled on the master server. The plug-in should be configured to enforce uniqueness in the replicated suffix. Because the master ensures that the values of the desired attribute are unique, you do not need to enable the plug-in on the consumer server.
Enabling the UID uniqueness plug-in on the consumer of a single master does not interfere with replication or normal server operations. However, it might cause slight performance degradation.
The UID uniqueness plug-in was not designed for use in a multimaster replication scenario. Because multimaster replication uses a loosely consistent replication model, simultaneously adding the same attribute value on both servers will not be detected, even if the plug-in is enabled on both servers.
However, you can use the UID uniqueness plug-in if the attribute on which you are performing the uniqueness check is a naming attribute, and the uniqueness plug-in is enabled for the same attribute in the same subtrees on all masters.
When these conditions are met, uniqueness conflicts are reported as naming conflicts at replication time. Naming conflicts must be resolved manually. For more information, refer to Solving Common Replication Conflicts.