Calendar Server performs multiple writes to user entries stored in Directory Server. The bulk of these writes occur when the user logs into Calendar Server for the first time and when the user performs certain actions. These actions include creating a calendar, subscribing to a calendar, changing a preference, and so on. If you do not take these actions into consideration, the Directory Master Server can experience heavy loads.
If you use Directory replication, the LDAP Master Server is replicating entries to the LDAP Replica servers. As Calendar users perform one of these actions, Calendar Server will only be able to write changes to the Master Directory Server. This is because the Replicas are read-only.
A second interaction consideration exists in these replicated Directory structures. As users make preference changes, their changes might not be rendered successful until the change is successfully replicated from the Master Directory Server to the Directory Replica, which is in use by the Calendar Server. A workaround is available, in which you configure Calendar Express (cshttpd) attempts to cache the change locally to avoid this latency delay. See Planning for the Calendar Server LDAP Data Cache for more information.