When you add a host to a vSphere HA cluster, an agent is uploaded to the host and configured to communicate with other agents in the cluster. Each host in the cluster functions as a master host or a slave host.
When vSphere HA is enabled for a cluster, all active hosts (those not in standby or maintenance mode, or not disconnected) participate in an election to choose the cluster's master host. The host that mounts the greatest number of datastores has an advantage in the election. Only one master host typically exists per cluster and all other hosts are slave hosts. If the master host fails, is shut down or put in standby mode, or is removed from the cluster a new election is held.
The master host in a cluster has a number of responsibilities:
Monitoring the state of slave hosts. If a slave host fails or becomes unreachable, the master host identifies which virtual machines need to be restarted.
Monitoring the power state of all protected virtual machines. If one virtual machine fails, the master host ensures that it is restarted. Using a local placement engine, the master host also determines where the restart should be done.
Managing the lists of cluster hosts and protected virtual machines.
Acting as vCenter Server management interface to the cluster and reporting the cluster health state.
The slave hosts primarily contribute to the cluster by running virtual machines locally, monitoring their runtime states, and reporting state updates to the master host. A master host can also run and monitor virtual machines. Both slave hosts and master hosts implement the VM and Application Monitoring features.
One of the functions performed by the master host is to orchestrate restarts of protected virtual machines. A virtual machine is protected by a master host after vCenter Server observes that the virtual machine's power state has changed from powered off to powered on in response to a user action. The master host persists the list of protected virtual machines in the cluster's datastores. A newly elected master host uses this information to determine which virtual machines to protect.
If you disconnect a host from a cluster, all of the virtual machines registered to that host are unprotected by vSphere HA.