1. A directory server system having a master server and a non-master server, wherein each server is configured to store directory server data comprising updates; said master server configured to: determine selected updates to the directory server data to send to the non-master server; send the selected updates to the non-master server asynchronously, wherein sending the updates asynchronously comprises sending each update to the non-master server without waiting for confirmation from the non-master server that previously sent updates have been completed at the non-master server; said non-master server having: a dependency checker operable for determining whether each update received from the master server depends on other updates among the selected updates, and if not for sending the update to an update application function, the update application function being operable for applying the updates received from the dependency checker in the non-master server directory server data; wherein the master server and the non-master server are both servers for the directory server data.