vSphere HA clusters enable a group of ESXi hosts to work together so that a higher level of availability is provided for virtual machines, which is more than what a single ESXi host can provide. vSphere HA pools ESXi hosts and virtual machines into a cluster. These ESXi hosts in the cluster are monitored and in the event of a failure, the virtual machines are restarted on an alternate ESXi host.
vSphere HA has recently undergone a complete overhaul and operates very differently from previous versions in vSphere 5.x.
An agent, Fault Domain Manager (FDM), is uploaded to the ESXi hosts as they are added to a vSphere HA cluster. This agent is configured to communicate with other agents within the vSphere HA cluster. An ESXi host can function as either a master host or a slave host.
When a cluster is enabled for vSphere HA, all active ESXi hosts participate in a process known as election. The election will determine the master host in the cluster; there is one master...