 | Documentation Feedback |  |
This section discusses Auto Scaling behavior with AutoScalingGroup size rebalancing and instance usage.
Amazon Auto Scaling attempts to distribute instances evenly between the Availability Zones that are enabled for your AutoScalingGroup. Auto Scaling uses the Availability Zone with the least number of instances when launching new instances. However, if an Availability Zone has insufficient capacity or if Amazon EC2 is unable to launch new instances in it, then Auto Scaling launches instances in another Availability Zone to satisfy the required capacity for your group.
Certain operations and conditions can cause your AutoScalingGroup to become unbalanced. Auto Scaling compensates by creating a rebalancing activity under any of the following conditions:
You issue a request to change the Availability Zones for your group.
You call TerminateInstanceInAutoScalingGroup, which causes the group to become unbalanced.
An Availability Zone that previously had insufficient capacity recovers and has additional capacity available.
Auto Scaling always launches new instances before attempting to terminate old ones, so a rebalancing activity will not compromise the performance or availability of your application.
Because Auto Scaling always attempts to launch new instances before terminating old ones, being at or near the specified maximum capacity could impede or completely halt rebalancing activities. To avoid this problem, the system can temporarily exceed the specified maximum capacity of a group by a 10% margin during a rebalancing activity. The margin is only extended if the group is at or near maximum capacity and needs rebalancing (either as a result of user-requested rezoning or to compensate for zone availability issues). The extension only lasts as long as needed to re-balanced the group (typically a few minutes).
