How can I make sure ASG is scaling EC2 instances in a correct Zone sequence, i.e when I scale ASG from 3 instances to 5 instances, it needs to have 2 nodes in Zone-A, 2 in Zone-B and 1 in Zone-C. But in our case, it ends up in 2 nodes in Zone-A, 1 node in Zone-B and 2 nodes in Zone-C.
How can I scale EC2 instances in ASG in Zone Sequence
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AWS ASG launches new instances in all Availability zones you enabled for that particular ASG. This is an extract from the official documentation.
Amazon EC2 Auto Scaling attempts to distribute instances evenly between the Availability Zones that are enabled for your Auto Scaling group. Amazon EC2 Auto Scaling does this by attempting to launch new instances in the Availability Zone with the fewest instances. If the attempt fails, however, Amazon EC2 Auto Scaling attempts to launch the instances in another Availability Zone until it succeeds
If you increase the desired capacity to say 9 (and you have 3 AZ's), you'll see there's a high chance there will be 3 instances on each AZ.
There is no way to control which AZ the AutoScaling Group will launch instances in.
The only work around I can think of is that you could make 1 ASG per AZ and then control the desired on your own via a script instead of using a scaling policy. I would recommend trying to make sure your application is as ephemeral as possible without zonal dependencies so that instances can be added in any zone