When a directory server connects to a replication server, the replication server must determine how up to date the directory server data is before the replication server can send changes that the directory server has not yet seen. This “up to date” state of the directory server is called the replication server state.
Server state is maintained as a vector. A server might have missed relatively old changes from another remote server, yet might already have seen and processed more recent changes from a server that is close by. Server state is therefore maintained by recording the last change number processed by each replica, according to the replica identifier.
Because administrators can stop and restart servers, the server state must be saved to stable storage. Ideally saving the server state would be done after each local or replicated change is made. Saving information to the database after each change would add significant overhead, however. Server state is therefore kept in memory and saved to the database on a regular basis, and when the server is properly shut down.
A drawback of this approach is that brutal interruptions such as kills and crashes can cause the server to lose track of changes that have already been processed. This can result in the need to fix inconsistencies when the server restarts. For an explanation of how crash recovery is managed, see Directory Server Crashes.