Because email is a mission-critical application, Microsoft has
invested a lot of engineering talent as well as money over the
years to provide Microsoft Exchange Server with the ability to
resist different types of failure and deliver a highly available
service. Exchange Server 2007 was a watershed for high
availability in many ways because of the introduction of log
replication technology in local continuous replication (LCR),
cluster continuous replication (CCR), and standby continuous
replication (SCR). Now Exchange Server 2010 takes a new approach
to high availability by introducing the Database Availability
Group (DAG), which is based on many of these same log
replication techniques.  However, working with DAGs introduces
new concepts, design challenges, and operational concerns that
administrators have to understand before bringing a DAG into
production. This article covers the underlying concept and
explains Microsoft's motivation for the introduction of DAGs in
Exchange 2010. A future article from Pau