My Auto Scaling group scaled down without my intervention. Why did this happen?
Scale-downs are user initiated, or triggered by configured scale-down policies and scheduled scaling. When a scale-down occurs, an instance is terminated according to the configured termination policy.
View your Auto Scaling group scaling history in the Amazon EC2 console, using the AWS Command Line Interface (AWS CLI), or using the AWS API.
If your Auto Scaling group scales down because of a scale-down policy, a message similar to the following appears in the scaling history:
At 2016-05-08T13:55:14Z a monitor alarm My-Scale-Down-Alarm in state ALARM
triggered policy Decrease Group Size changing the desired capacity from 4 to 3.
You can adjust your Auto Scaling group scaling policy using the Amazon EC2 console or with the AWS CLI put-scaling-policy command. To configure when a scale-down occurs, adjust the associated Amazon CloudWatch alarm. Or, create a new alarm and then associate the new alarm with the Auto Scaling group scaling policy.
User-initiated scale down
A scale-down triggered by a user displays an event similar to the following in the scaling history:
At 2016-05-13T15:03:47Z a user request update of AutoScalingGroup constraints
to min: 12, max: 20, desired: 13 changing the desired capacity from 14 to 13.
Note: CloudTrail must be configured and enabled before you can begin recording API calls.
A scale-down initiated by a scheduled scale down action displays an event similar to the following in the scaling history:
At 2016-02-12T16:01:25Z a scheduled action update of AutoScalingGroup constraints
to min: 1, max: 5, desired: 2 changing the desired capacity from 3 to 2. At
2016-02-12T16:01:25Z the scheduled action ScaleDown executed. Setting max size
from 1 to 5. Setting desired capacity from 3 to 2.
To view scheduled scaling using the AWS CLI, run the following command. Replace MY-ASG-NAME with the name of your Auto Scaling group.