VMware Fault Tolerance provides continuous availability to applications running in a virtual machine, preventing downtime and data loss in the event of server failures.
VMware Fault Tolerance, when enabled for a virtual machine, creates a live shadow instance of the primary, running on another physical server.
The two instances are kept in virtual lockstep with each other using VMware vLockstep technology
The two virtual machines play the exact same set of events, because they get the exact same set of inputs at any given time.
The two virtual machines constantly heartbeat against each other and if either virtual machine instance loses the heartbeat, the other takes over immediately. The heartbeats are very frequent, with millisecond intervals, making the failover instantaneous with no loss of data or state.
VMware Fault Tolerance requires a dedicated network connection, separate from the VMware VMotion network, between the two physical servers.