A. Exchange 2010 has taken the Exchange 2007 Cluster Continuous Replication (CCR) and Standby Continuous Replication (SCR) and combined them into a new high availability model which allows 16-node Database Availability Groups (DAGs).

You can have up to 16 mailbox servers in a DAG. A DAG uses failover clustering behind the scenes, but you never actually touch any cluster configuration. Instead of the server being the unit of failover, the mailbox database becomes the unit of failover and is managed entirely within Exchange. A new component, Active Manager, is the high availability brain for Exchange 2010. It manages which copy of the database should be active and which should be passive.

Active Manager uses Active Directory (AD) as the container for configuration information. Active Manager runs on every server in the DAG. In the event that the current primary copy of a database fails, Active Manager selects the best copy to make primary based availability and the least potential data loss.

Exchange 2010 no longer has the concept of storage groups. It has databases that have a corresponding transaction log and you can have up to 50 databases on each mailbox server. Now you just select which servers host a copy of each database. (The best practice is to have at least three servers with a copy of each database.) A new requirement in Exchange 2010 is that each database's name must be unique throughout the entire Exchange organization. An example DAG structure is shown here.

Click to expand.

The connectivity via the Client Access server (CAS) means that servers in the DAG aren't restricted to being in the same IP subnet or AD site. You can have servers in a DAG anywhere in your environment. To support this new model and mailbox failover, all clients (even MAPI, which previously connected directly to the mailbox server) now connects using CAS (connecting via CAS is called MAPI on the Middle Tier).

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