Load balancing is often used to implement failover — the continuation of a service after the failure of one or more of its components. The components are monitored continually (e.g., web servers may be monitored by fetching known pages), and when one becomes non-responsive, the load balancer is informed and no longer sends traffic to it. And when a component comes back on line, the load balancer begins to route traffic to it again. For this to work, there must be at least one component in excess of the service’s capacity. This is much less expensive and more flexible than failover approaches where a single “live” component is paired with a single “backup” component that takes over in the event of a failure. In a RAID disk controller, using RAID1 (mirroring) is analogous to the “live/backup” approach to failover, where RAID5 analogous to load balancing failover.
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