I need to determine why my Auto Scaling group scaled down.

Scale downs can be triggered by configured scale down policies, scheduled scaling, or user action. Whenever a scale down occurs, the instance is terminated according to the configured termination policy.

View the scaling history of your Auto Scaling group in the AWS console or use the AWS Command Line Interface (CLI) or the AWS API as described here.

A scale down as a result of a scale-down policy causes a message similar to the following 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 AWS console or with the AWS CLI put-scaling-policy command. To configure when a scale down occurs, adjust the associated CloudWatch alarm or create a new alarm and associate the new alarm with the Auto Scaling group scaling policy.

A scale down triggered by a user causes a message 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.

You can determine the user that made the API call by viewing your CloudTrail logs.

Note
CloudTrail must be configured and enabled before you can begin recording API calls.

A scale down initiated by a scheduled scale-down action causes a message 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.

Scheduled scaling can be configured by using the AWS CLI. To view scheduled scaling, run the following command, substituting the appropriate value for MY-ASG-NAME:

aws autoscaling describe-scheduled-actions --auto-scaling-group-name MY-ASG-NAME

For more information about schedule-based scaling, see Scheduled Scaling.

Auto Scaling group, scale down, schedule, constraints, CloudWatch, alarm, policy, termination


Did this page help you? Yes | No

Back to the AWS Support Knowledge Center

Need help? Visit the AWS Support Center.