Exchange Server 2007 introduced a built in data replication technology called continuous replication. Continuous replication, which was available in three forms: local, cluster, and standby, significantly reduced the cost of deploying a highly available Exchange infrastructure, and provided a much improved deployment and management experience over previous versions of Exchange. Even with these cost savings and improvements, however, running a highly available Exchange 2007 infrastructure still required a great deal of time and expertise because the integration between Exchange and Windows failover clustering wasn't seamless. In addition, customers wanted an easier way to replicate their e-mail data to a remote location, in order protect their Exchange environment against site-level disasters.
Exchange 2010 uses the same continuous replication technology found in Exchange 2007. Exchange 2010 combines on-site data replication (CCR) and off-site data replication (SCR) into a single framework called a database availability group (DAG). Once servers have been added to a DAG, administrators can add replicated database copies incrementally (up to 16 total), and Exchange 2010 switches between these copies automatically, as needed, to maintain availability.
Unlike Exchange 2007, where clustered mailbox servers required dedicated hardware, Mailbox servers in a DAG can host other Exchange roles (Client Access, Hub Transport, Unified Messaging), providing full redundancy of Exchange services and data with just two servers.
This new high availability architecture also provides simplified recovery from a variety of failures (disk-level, server-level, and datacenter-level), and it can be deployed on a variety of storage types.
For more information about DAGs, see Managing Database Availability Groups.
Active Manager
DAGs use a new component in Exchange 2010 called Active Manager. Active Manager includes functionality that replaces the failover management features provided by the Cluster service in previous versions of Exchange.
In Exchange 2010, the Microsoft Exchange Replication service periodically monitors the health of all mounted databases. In addition, it also monitors Extensible Storage Engine (ESE) for any I/O errors or failures. When the service detects a failure, it notifies Active Manager. Active Manager then determines which database copy should be mounted and what it required to mount that database. In addition, tracks the active copy of a mailbox database (based on the last mounted copy of the database) and provides the tracking results information to the RPC Client Access component on the Client Access server to which the client is connected.
When an administrator makes a database copy the active mailbox database, this process is known as a switchover. When a failure affecting a database occurs and a new database becomes the active copy, this process is known as a failover. This process also refers to a server failure in which one or more servers bring online the databases previously online on the failed server. When either a switchover or failover occurs, other Exchange 2010 server roles become aware of the switchover almost immediately and will redirect client and messaging traffic to the new active database.
For example, if an active database in a DAG fails because of an underlying storage failure, Active Manager will automatically recover by failing over to a database copy on another Mailbox server in the DAG. In the event the database is outside the automatic mount criteria and cannot be automatically mounted, an administrator can manually perform a database failover.