A simple asymmetric high availability system has two physical nodes. The primary node is usually active, with the other node acting as a backup node, ready to take over if the primary node fails. To accomplish a fail over, the shared disk array is switched so that it is mastered by the backup node. The Calendar Server processes are stopped on the failing primary node and started on the backup node.
There are several advantages of this type of high availability system. One advantage is that the backup node is dedicated and completely reserved for the primary node. This means there is no resource contention on the backup node when a failover occurs. Another advantage is the ability to perform a rolling upgrade; that is, you can upgrade one node while continuing to run Calendar Server software on the other node. Changes you make to the ics.conf file while upgrading the first node will not interfere with the other instance of Calendar Server software running on the secondary node because the configuration file is read only once, at startup. You must stop and restart the calendar processes before the new configuration takes effect. When you want to upgrade the other node, you perform a failover to the upgraded primary node and proceed with the upgrade on the secondary node.
You can, of course, choose to upgrade the secondary node first, and then the primary node.
The asymmetric high availability model also has some disadvantages. One disadvantage is that the backup node stays idle most of the time, making this resource underutilized. Another possible disadvantage is the single storage array. In the event of a disk array failure with a simple asymmetric high availability system, no backup is available