SQL Server - Always on - Replica

Question

I am reading about the sql offering, Always on and there is one thing which particularly messing with mind. Are replicas in Always on

1. Can exist in different geographical location ? Let's say i have 2 node (replica), primary replica exists in Atlanta (US) . is it possible to have secondary node in India (Delhi).

I am aware that nodes can in different states or location in India. lets say 1 in delhi and 2nd in Mumbai.

Is adding node or replica is restricted to some regional or geographical boundaries ?

Regards

Answer 1

If you want synchronous commit, yes, there a restriction, which is usually given as 100 km. The reason for this is that the transaction cannot be committed until the log has been hardened on the secondary replica. And the longer the distance, the longer the latency. If you wonder that defines this limit, it is simply the speed of light, 3E8 m/s.

For asynchronous commit, you could have setup like the above, but I can't say that it is recommendable. Rather, you would have one AG in Mumbai and one AG in Atlanta, and then you would join these two AG into a distributed AG.

And overall. since you normally have a Windows cluster for your AG, you probably want the cluster nodes to be close to each other for low latency.

Answer 2

Hi @sourabh tiwariAlwaysOn Distributed Availability Group (AlwaysOn DAG) runs on top of two distinct AGs meaning that they reside on two distinct WSFCs with their own quorum and voting management.

This configuration allows secondary replicas of an AG to exist in a different geographical region than the primary. An example use case would be to enable read-only workloads for remote regions and at the same time avoid any potential network problem at the secondary site which can affect the primary site.

Refer to this article for more details.

Best regards,

Cosmog Hong

---

If the answer is the right solution, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".

Note: Please follow the steps in our Documentation to enable e-mail notifications if you want to receive the related email notification for this thread.