The upper part of this picture shows a cluster where the left node is currently active, i.e., the service's IP address that the client machines are talking to is currently on the left node.

The service, including its IP address, can be migrated to the other node at any time, either due to a failure of the active node or as an administrative action. The lower part of the illustration shows a degraded cluster. In HA speak the migration of a service is called failover, the reverse process is called failback and when the migration is triggered by an administrator it is called switchover.

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