For VMs that need to be up 100 percent of the time and do not tolerate even a brief interruption caused by a reboot as a result of host failure, vSphere offers a continuous availability option called Fault Tolerance (FT).

FT creates and maintains an exact copy of a running virtual machine—secondary VM—on another host. Both VMs exchange heartbeats to monitor each other's status.

When a host with a primary VM fails, the secondary VM becomes active almost instantly. There is a little delay but clients do not see the interruption. As everything that happens on the primary VM replays on the secondary one, this failover happens transparently without the interruption of the existing network connections or in-progress transactions. The replication delay is typically less than 1 millisecond and it's unlikely that there will be transactions that have not yet been replicated. Such transactions, should they happen, will be lost.