Create a step scaling policy for scale in - Amazon EC2 Auto Scaling
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Create a step scaling policy for scale in

To create a step scaling policy for scale in for your Auto Scaling group, use one of the following methods:

Console
Step 1: Create a CloudWatch alarm for the metric low threshold

  1. Open the CloudWatch console at https://console.amazonaws.cn/cloudwatch/.

  2. If necessary, change the Region. From the navigation bar, select the Region where your Auto Scaling group resides.

  3. In the navigation pane, choose Alarms, All alarms and then choose Create alarm.

  4. Choose Select metric.

  5. On the All metrics tab, choose EC2, By Auto Scaling Group, and enter the Auto Scaling group's name in the search field. Then, select CPUUtilization and choose Select metric. The Specify metric and conditions page appears, showing a graph and other information about the metric.

  6. For Period, choose the evaluation period for the alarm, for example, 1 minute. When evaluating the alarm, each period is aggregated into one data point.

    Note

    A shorter period creates a more sensitive alarm.

  7. Under Conditions, do the following:

    • For Threshold type, choose Static.

    • For Whenever CPUUtilization is, specify whether you want the value of the metric to be less than or less than or equal to the threshold to breach the alarm. Then, under than, enter the threshold value that you want to breach the alarm.

      Important

      For an alarm to use with a scale in policy (metric low), make sure you do not choose greater than or greater than or equal to the threshold.

  8. Under Additional configuration, do the following:

    • For Datapoints to alarm, enter the number of data points (evaluation periods) during which the metric value must meet the threshold conditions for the alarm. For example, two consecutive periods of 5 minutes would take 10 minutes to invoke the alarm state.

    • For Missing data treatment, choose Treat missing data as bad (breaching threshold). For more information, see Configuring how CloudWatch alarms treat missing data in the Amazon CloudWatch User Guide.

  9. Choose Next.

    The Configure actions page appears.

  10. Under Notification, select an Amazon SNS topic to notify when the alarm is in ALARM state, OK state, or INSUFFICIENT_DATA state.

    To have the alarm send multiple notifications for the same alarm state or for different alarm states, choose Add notification.

    To have the alarm not send notifications, choose Remove.

  11. You can leave the other sections of the Configure actions page empty. Leaving the other sections empty creates an alarm without associating it to a scaling policy. You can then associate the alarm with a scaling policy from the Amazon EC2 Auto Scaling console.

  12. Choose Next.

  13. Enter a name (for example, Step-Scaling-AlarmLow-RemoveCapacity) and, optionally, a description for the alarm, and then choose Next.

  14. Choose Create alarm.

Use the following procedure to continue where you left off after creating your CloudWatch alarm.

Step 2: Create a step scaling policy for scale in

  1. Open the Amazon EC2 console at https://console.amazonaws.cn/ec2/, and choose Auto Scaling Groups from the navigation pane.

  2. Select the check box next to your Auto Scaling group.

    A split pane opens up in the bottom of the page.

  3. Verify that the scaling limits are appropriately set. For example, if your group's desired capacity is already at its minimum, you need to specify a new minimum in order to scale in. For more information, see Set scaling limits for your Auto Scaling group.

  4. On the Automatic scaling tab, in Dynamic scaling policies, choose Create dynamic scaling policy.

  5. For Policy type, choose Step scaling, and then specify a name for the policy.

  6. For CloudWatch alarm, choose your alarm. If you haven't already created an alarm, choose Create a CloudWatch alarm and complete step 4 through step 14 in the previous procedure to create an alarm.

  7. Specify the change in the current group size that this policy will make when executed using Take the action. You can remove a specific number of instances or a percentage of the existing group size, or set the group to an exact size.

    For example, to create a scale in policy that decreases the capacity of the group by two instances, choose Remove, enter 2 in the next field, and then choose capacity units. By default, the upper bound for this step adjustment is the alarm threshold and the lower bound is negative (-) infinity.

  8. To add another step, choose Add step and then define the amount by which to scale and the lower and upper bounds of the step relative to the alarm threshold.

  9. Choose Create.

Amazon CLI

To create a step scaling policy for scale in (decrease capacity), you can use the following example commands. Replace each user input placeholder with your own information.

When you use the Amazon CLI, you first create a step scaling policy that provides instructions to Amazon EC2 Auto Scaling about how to scale in when a metric's value is decreasing. Then, you create the alarm by identifying the metric to watch, defining the metric low threshold and other details for the alarms, and associating the alarm with the scaling policy.

Step 1: Create a policy for scale in

Use the following put-scaling-policy command to create a step scaling policy named my-step-scale-in-policy, with an adjustment type of ChangeInCapacity that decreases the capacity of the group by 2 instances when the associated CloudWatch alarm breaches the metric low threshold value.

aws autoscaling put-scaling-policy \
  --auto-scaling-group-name my-asg  \
  --policy-name my-step-scale-in-policy \
  --policy-type StepScaling \
  --adjustment-type ChangeInCapacity \
  --step-adjustments MetricIntervalUpperBound=0.0,ScalingAdjustment=-2

Record the policy's Amazon Resource Name (ARN). You need it to create the CloudWatch alarm for the policy. 

{
    "PolicyARN": "arn:aws-cn:autoscaling:region:123456789012:scalingPolicy:ac542982-cbeb-4294-891c-a5a941dfa787:autoScalingGroupName/my-asg:policyName/my-step-scale-out-policy
}
Step 2: Create a CloudWatch alarm for the metric low threshold

Use the following CloudWatch put-metric-alarm command to create an alarm that decreases the size of the Auto Scaling group based on average CPU threshold value of 40 percent for at least two consecutive evaluation periods of two minutes. To use your own custom metric, specify its name in --metric-name and its namespace in --namespace.

aws cloudwatch put-metric-alarm --alarm-name Step-Scaling-AlarmLow-RemoveCapacity \
  --metric-name CPUUtilization --namespace AWS/EC2 --statistic Average \
  --period 120 --evaluation-periods 2 --threshold 40 \
  --comparison-operator LessThanOrEqualToThreshold \
  --dimensions "Name=AutoScalingGroupName,Value=my-asg" \
  --alarm-actions PolicyARN