There is set of rules to terminate instance for Auto Scaling when we have multiple AZ.. Same way if we wanted scale up if we have multiple available zones, where exactly instances will be created .. is there any hierarchy?

CodePudding user response：

According to aws docs, if you have multiple availabilty zones for an autoscaling group, aws try to distribute the instance in evenly manner. So if your desired capacity is 8 and there are 4 instances in az-1 and 3 in az-2, the remaining one instance will be created in az-2.

When one Availability Zone becomes unhealthy or unavailable, Amazon EC2 Auto Scaling launches new instances in an unaffected Availability Zone. When the unhealthy Availability Zone returns to a healthy state, Amazon EC2 Auto Scaling automatically redistributes the application instances evenly across all the Availability Zones 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 in other Availability Zones until it succeeds.

You can read more about this here.