Datacenter Activation Coordination mode (DAC mode) is a feature of Exchange database availability groups that is designed to prevent split brain scenarios.
Within a database availability group (DAG) each database can have one active copy, and up to 15 passive copies at any given time. Changes that occur in the active database copy are replicated to the passive copies by a process of continuous replication. The active copy can be dismounted, and one of the passive copies mounted to become the active copy, when a switchover or failover occurs.
In a split brain scenario, two copies of a database would be active at the same time, mounted on DAG members that are unable to communicate with each other due to a network problem, causing the databases to diverge. This is a situation that must be avoided, because it creates a difficult recovery scenario and will likely result in data loss.
DAC mode enables a protocol called Datacenter Activation Coordination Protocol (DACP). You can read an example of how DAC mode and DACP work to avoid split brains in this article.
The recommended practice for Exchange Server DAGs is to enable DAC mode if:
- The DAG has more than one member (DAGs start with zero members, and can have one member)
- Third party replication mode is not enabled, or the third party replication vendor specifies that DAC mode can be used
In addition to preventing split brains, DAC mode enables the use of Exchange site-resilience cmdlets (Stop-DatabaseAvailabilityGroup, Restore-DatabaseAvailabilityGroup, and Start-DatabaseAvailabilityGroup). Those cmdlets are used perform datacenter switchovers.
To review the DAC mode configuration for a DAG, use the Get-DatabaseAvailabilityGroup cmdlet.
To enable DAC mode, use the Set-DatabaseAvailabilityGroup cmdlet.
DAC mode can be enabled for DAGs that exist within a single datacenter location. A multi-site DAG is not required. Although less likely, a split brain can still occur for DAGs within a single datacenter location under the right conditions.