Exploring the Differences Between Always On Availability Groups and Distributed Availability Groups (DAGs) in SQL Server

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 A distributed availability group (AG) is a special type of availability group that spans two separate availability groups. Distributed availability groups are available starting with SQL Server 2016.

The availability groups that participate in a distributed availability group don't need to be in the same location. They can be physical, virtual, on-premises, in the public cloud, or anywhere that supports an availability group deployment.

The following figure shows a high-level view of a distributed availability group that spans two availability groups (AG 1 and AG 2), each configured on its own WSFC. The distributed availability group has a total of four replicas, with two in each availability group. Each availability group can support up to the maximum number of replicas, so a distributed availability can have up to 18 total replicas.

 

 

 However, data movement is slightly different within distributed availability groups compared to a traditional availability group. Although each availability group has

a primary replica, there is only one copy of the databases participating in a distributed availability group that can accept inserts, updates, and deletions.

As shown in the following figure, AG 1 is the primary availability group. Its primary replica sends transactions to both the secondary replicas of AG 1 and the primary replica of AG 2. The primary replica of AG 2 is also known as a forwarder. A forwarder is a primary replica in a secondary availability group in a distributed availability group. The forwarder receives transactions from the primary replica in the primary availability group and forwards them to the secondary replicas in its own availability

 



The only way to make AG 2's primary replica accept inserts, updates, and deletions, is to manually fail over the distributed availability group from AG 1. In the preceding figure, because AG 1 contains the writeable copy of the database, issuing a failover makes AG 2 the availability group that can handle inserts, updates, and deletions.

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