Before learning what
continuous replication is, you need to understand what clustering was like in previous versions.
Exchange clustering is built on top of Windows Server 2003 Server Clustering (WSSC). WSSC clusters
allow for multiple nodes, or servers, to share a set of resources. In Exchange, the database and log files
are examples of shared resources. Also, Exchange creates a virtual Exchange server, called the Clustered
Mailbox Server (CMS). The CMS is what mail clients use as their mailbox server. The clustering service
is able to detect a fault in the network, node, or storage and tries to move the resources to the operational
node. Exchange supports up to eight nodes in a Single Copy Cluster, where at least one node is a
passive node. A passive node is a server node in the cluster that is simply waiting for a failure on an
active node so that it can take ownership of the resources and restore service. Mail clients have no idea
that the CMS is not a ??? real ??? server and will simply reconnect when the resources are brought online on
the passive node.
Another piece of the WSSC cluster is that the nodes need to make sure only one server has ownership
of the resources at a time. This is achieved through the use of a quorum resource, which is shared disk.
The quorum resource is also shared across all the nodes and contains cluster state and configuration
information.
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