Exchange must provide Mailbox databases in a highly available and redundant configuration.

To protect Exchange Server mailbox databases and the data they contain by configuring Mailbox servers and databases for high availability and site resilience. A database availability group (DAG) is a component of the Mailbox server high availability and site resilience framework built into Microsoft Exchange Server 2013. A DAG is a group of Mailbox servers that hosts a set of databases and provides automatic database-level recovery from failures that affect individual servers or databases. A DAG is a boundary for mailbox database replication and database and server switchovers and failovers. Any server in a DAG can host a copy of a mailbox database from any other server in the DAG. When a server is added to a DAG, it works with the other servers in the DAG to provide automatic recovery from failures that affect mailbox databases, such as a disk, server, or network failure.