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Generally speaking, there are 2 types of SQL high availability clusters, namely Mirroring Cluster and Replication and MirroringCluster.
SQL Mirroring Cluster
In a Mirroring Cluster, there is an active SQL database server and a passive database server. When the active database server is down for some reason, the cluster will fail over to the passive database server, and the passive database will become the active database server.