What is VMware HA and how does it work?

VMware HA is a feature which is available in vCenter to provide highly available virtual machines.

It allows virtual machines and the application(s) running inside them to gain a level of high availability that may otherwise have no failover option.
In the event of a physical ESX/ESXi server any virtual machines running are restarted on another physical server.

You may have a legacy application running on a server that is mission critical but to make it highly available would be costly or not possible due to application limitations.
By running the application inside a virtual machine and leveraging the HA feature you can ensure that virtual machine is only unavailable for a minimal amount of time.

The following is required for HA:
Two or more ESX/ESXi Servers in a HA cluster
HA licensed on the ESX/ESXi Servers in the HA cluster
Central storage (typically a SAN) for the virtual machine files which is available to all ESX/ESXi Servers in the HA cluster
The same network available to all ESX/ESXi Servers in the HA cluster

VMware HA works by monitoring all ESX/ESXi servers in the HA cluster via an agent, maintaining a heartbeat.
When this heartbeat is lost such as in the event of a server failure the virtual machine is started on another server. This is done automatically.
Because the virtual machines files are stored on a central storage device using the VMFS clustered file system, the virtual machine files can be accessed and started on another server.

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