There are different ways to obtain high availability for a Directory Service. One is to deploy Directory Server in a Cluster. Another one is to deploy 2 instances of the Directory Server and have them replicate to each other in a multi-master replication topology.
But even when using a hardware load balancer, you may want to front your Directory Server instances with a pair of load-balanced Directory Proxy Servers.
This way, you have physical redundancy at the load balancer level, and intelligent LDAP-aware load balancing at the proxy server level.
Directory Proxy Server 6 is very nice in that you can split binds, searches, and updates amongst several DS instances, and the connection state is maintained by the proxy, not the DS instance. So if a DS instance fails or is stopped for maintenance, the application isn’t forced to rebind, the proxy fails-over to another DS for writing or reading (*).
With Solaris 10 and Solaris Containers, you can do this on a pair of systems, each with a zone for a replicated Master DS, and another zone each for a DPS instance. The DPS instances are configured to load balance the authentication,read, write operation among the DS master zones.
This works out very well for some of our customers.
* – The failover does not work for searches that are already in progress and for which entries have been returned already. And it does not work either with Persistent searches.