The vSphere HA mechanism is essentially made up of three different but equally important components:

In vSphere 5.0, the FDM replaced the Legato Automated Availability Manager (AAM). The FDM agent runs on ESXi hosts and communicates with the other hosts in the cluster with regard to available resources and the state of running virtual machines. It is also responsible for the heartbeat mechanism, the placement of virtual machines, and the restarting of VMs in a vSphere HA event in relation to the hostd agent. The FDM communicates directly with both the hostd agent and the vCenter Server. The hostd agent, which runs on the host server, is required for vSphere HA to function correctly. If the hostd agent is not functioning, the FDM puts all vSphere HA functions on hold until the agent is operational again. The vCenter Server deploys and manages the FDM agents after a host is added to an HA cluster. The vCenter server also provides all the configuration changes within a cluster to the host elected as the master.