AG Group failover and quorum We have a 4 node AG set up across two data centers using 4 replicas with a file share witness. Nodes 1 and 2 are in data center 1 and Nodes 3 and 4 in data center 2. Node 1 is primary and is set to synchronous commit with automatic failover to node 2 in the same datacenter. Node 1 is set to asynchronous commit to nodes 3 and 4 in data center 2. Node weighting is set to one for nodes 1 and 3 (main node in each data center) and zero for nodes 2 and 4.

Please, can someone tell me what the behavior would be if we had an unexpected SQL Server shut down at node 1? 

Would the AG attempt to failover to node 3 in data center 2 due to the node weighting, fail and not attempt to fail over to any other replicas. Or would the AG group ignore node 3 due to it being async and try to fail over to the first synchronous replica with automatic failover based on the preferred owner's list?
The configuration described in the original topic is a bit strange. There are exists two resource groups SYSTEM (CNO, CNO_VIP1, CNO_VIP2, FSW) and AG (LISTENER, LISTENER_VIP1, LISTENER_VIP2, AG).

These two resource groups can be owned by different WSFC nodes. For example SYSTEM is owned by node from DC2 and AG is owned by node from DC1. In this case, if the link between DC1 and DC2 is broken:


Then DC1 will have only 1 vote (NODE1)
Then DC2 will have only 2 votes (NODE3 + FSW)


Accordingly to the quorum configuration DC2 part will alive (due to majority of votes) and DC1 will die (due to minority of votes). But as soon as synchronization between DC1 and DC2 is asynchronous there won't be automatic failover to DC2.

So you will end up with the situation when DC2 is alive but can not handle DB workload, because AG will wait manual intervention (allow_data_loss).

To avoid such situation, you need to assign votes like that

DC1:


NODE1: 1
NODE2: 1


ASYNC SYNCHRONIZATION

DC2


NODE3: 0
NODE4: 0


In that configuration AG will fail over to the NODE2 (DC1).

You'd better to explain the purpose of DC2. Is it disaster recovery DC or you need it for read scale or both?