going though Allan Hirt’s 24 SQLPASS recording, he has explained the new feature of SQL Server Availability group you can get it here.
he is expert in Clustering /AG/ and published several blogs and books.
Distributed AG is advance and extended to AG its like AG to AG
one AG which will be on WSFC between two instances.
WSFC 1 :Instance A is AG with Instance B and
WSFC 2 :Instance C is AG with Instance D
so we can perform Distributed AG from Instance A to Instance C
then all four instances are in sync with each other. but it has some issues if we make it sync. so its recommendation to make WSFC 1 to WSFC 2 with async so that things will be working efficiently else Instance A will be quite slow to get sync all.
It has its limitations:
important limitation is we can configure it only with T-SQL queries. not with GUI and Powershell(this may overcome in future release)
supported in Enterprise edition only.
Automatic fail over to secondary AG is not supported for now
Thanks Allan 🙂