Fail over means switching to another machine when ONE of the machine fails. 

Fail over is a important technique in achieving high AVAILABILITY. Typically a load balancer is configured to fail over to another machine when the main machie fails. 

To achieve least down time, most load balancer SUPPORT a feature of heart beat check. This ensures that target machine is responding. As soon as a hear beat SIGNAL fails, load balancer stops SENDING request to that machine and redirects to other machines or cluster.

Fail over means switching to another machine when one of the machine fails. 

Fail over is a important technique in achieving high availability. Typically a load balancer is configured to fail over to another machine when the main machie fails. 

To achieve least down time, most load balancer support a feature of heart beat check. This ensures that target machine is responding. As soon as a hear beat signal fails, load balancer stops sending request to that machine and redirects to other machines or cluster.