The Exchange development team have done a nice job of expanding the high availability options with the 2007 release. With Exchange 2003, the only real HA design was to use what is now known as a Single Copy Cluster (SCC) - ie. there's one copy of the databases and log files, held on a Storage Area Network, and multiple physical cluster nodes connect to that SAN. Exchange 2007 introduced Local Continuous Replication and Cluster Continuous Replication, and is due to add Standby Continuous Replication later this year.

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Microsoft, Exchange 2003, Exchange 2007, Clustering