There is a section on how to monitor the Virtual Directory Server health via a hardware load-balancer. The principle is the same when monitoring Sun Directory Server instances.
Note: Most load-balancers also support simplistic health-check mechanisms, either based on the "ping" availability of a server, or a simple TCP connect. This health-check mechanism must NOT be used, because it is not reliable enough to verify whether any Virtual Directory Server instance is truly running. Instead, one of the two previously listed methods should be used.
HTTP GET and LDAP Search are recommended.
In a highly available deployment, there is usually a hardware load-balancer in front of the multiple Sun Directory Server instances. Similar deployment. Similar health-check should be applied as well.