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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. 

In a Mirroring Cluster, only one database server is active at one time and the database operation is served from the active server only. In principle, data change in the active database is copied to the passive database in real time. 

SQL Replication Cluster

In a Replication Clusters, there are multiple SQL database servers and all of them are active at one time. Data change in any database is replicated to the other databases in real time.