VMware High Availability (HA) is a technology that provides cluster-based monitoring of virtual machines running on included ESXi hosts. If a failure of the storage, network, or host occurs, the remaining ESXi hosts coordinate to restart the affected virtual machines on unaffected hosts. Through a variety of network and storage-based heartbeating, the ESXi hosts can detect and respond to a variety of failure or isolation events to provide the fastest automated recovery of virtual machines.
VMware HA offers a solution to protect applications running inside virtual machines that do not offer application-based high-availability or application cluster configurations. VMware HA, however, does not preclude the use of those features if the applications offer them. They can actually offer additional benefits on top of one another, though a detailed discussion on that topic is beyond the scope of this paper.
VMware HA is made possible via shared storage. Without shared storage, VMs and their data cannot be seen by surviving hosts and therefore a disaster restart operation is not an option. For this reason, it is a general best practice to provision storage identically and simultaneously to all hosts in a cluster—this provides the ability for any host to host any virtual machine on any other host in the cluster.