If you are looking for a software based hot fail over solution for directory server, consider using multi-master replication. Really. Even if you are one of those who cannot get along with the characteristics of loosely-consistent multi-master replication models, MMR can provide the best of both worlds if configured appropriately. Here’s the secret: despite allowing up to four masters, not one of those masters will force your clients to use it. In other words, you use one of the masters as your write master which is assigned the live virtual ip. Set up your heart-beat script to flip the virtual ip to one of the other masters when a problem is detected. Then you can work on bringing up the failed master. You won’t need to script any complicated consistency recovery mechanisms since the multi-master replication logic will recover consistency for you - that is what it is designed to do.

Using this topology you will have all the benefits of a single master plus up to three ready to go backup masters.