Two or more servers are configured in a cluster arrangement to operate
as a "virtual" SQL server. Each server has software locally installed AND also installed via a shared sub-system with the other servers in the
cluster.
A "heartbeat" is heard between servers in the cluster, ensuring each
server is functioning properly; if not, then "Failure" Occurs - i.e if one machine in the cluster fails, another assumes the responsibilities of the downed server thus the hardware failure does not result in loss of functionality for the application. (Voyager)
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