In terms of the SPOFs that are described at the beginning of this chapter, redundancy and clustering handle failure in the following ways:
Single hardware failure. In a clustered environment, this kind of failure has no impact on the directory service. Only multiple hardware failures impact the service in a cluster.
A single hardware failure is fatal to a machine that is not in a clustered environment. Therefore, even if you have redundant hardware, manual intervention is required to repair the failure.
Directory Server or Directory Proxy Server failure. In a clustered environment, the server is automatically restarted. Software failure must occur multiple times in quick succession to trigger the service group to switch to another node in the cluster. This handling of a software failure is also true in a redundant environment.
Database corruption. A cluster cannot survive this kind of failure. Depending on the architecture, a redundant solution should be able to survive database corruption.