Scale Spot Fleet using step scaling policies - Amazon Elastic Compute Cloud

Scale Spot Fleet using step scaling policies

With step scaling policies, you specify CloudWatch alarms to trigger the scaling process. For example, if you want to scale out when CPU utilization reaches a certain level, create an alarm using the CPUUtilization metric provided by Amazon EC2.

When you create a step scaling policy, you must specify one of the following scaling adjustment types:

  • Add – Increase the target capacity of the fleet by a specified number of capacity units or a specified percentage of the current capacity.

  • Remove – Decrease the target capacity of the fleet by a specified number of capacity units or a specified percentage of the current capacity.

  • Set to – Set the target capacity of the fleet to the specified number of capacity units.

When an alarm is triggered, the automatic scaling process calculates the new target capacity using the fulfilled capacity and the scaling policy, and then updates the target capacity accordingly. For example, suppose that the target capacity and fulfilled capacity are 10 and the scaling policy adds 1. When the alarm is triggered, the automatic scaling process adds 1 to 10 to get 11, so Spot Fleet launches 1 instance.

When a Spot Fleet terminates an instance because the target capacity was decreased, the instance receives a Spot Instance interruption notice.


The Spot Fleet request must have a request type of maintain. Automatic scaling is not supported for requests of type request, or Spot blocks.

  • Consider which CloudWatch metrics are important to your application. You can create CloudWatch alarms based on metrics provided by AWS or your own custom metrics.

  • For the AWS metrics that you will use in your scaling policies, enable CloudWatch metrics collection if the service that provides the metrics does not enable it by default.

To create a CloudWatch alarm
  1. Open the CloudWatch console at

  2. In the navigation pane, choose Alarms.

  3. Choose Create alarm.

  4. On the Specify metric and conditions page, choose Select metric.

  5. Choose EC2 Spot, Fleet Request Metrics, select a metric (for example, TargetCapacity), and then choose Select metric.

    The Specify metric and conditions page appears, showing a graph and other information about the metric you selected.

  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.


    A shorter period creates a more sensitive alarm.

  7. For Conditions, define the alarm by defining the threshold condition. For example, you can define a threshold to trigger the alarm whenever the value of the metric is greater than or equal to 80 percent.

  8. Under Additional configuration, for Datapoints to alarm, specify how many datapoints (evaluation periods) must be in the ALARM state to trigger the alarm, for example, 1 evaluation period or 2 out of 3 evaluation periods. This creates an alarm that goes to ALARM state if that many consecutive periods are breaching. For more information, see Evaluating an alarm in the Amazon CloudWatch User Guide.

  9. For Missing data treatment, choose one of the options (or leave the default of Treat missing data as missing). For more information, see Configuring how CloudWatch alarms treat missing data in the Amazon CloudWatch User Guide.

  10. Choose Next.

  11. (Optional) To receive notification of a scaling event, for Notification, you can choose or create the Amazon SNS topic you want to use to receive notifications. Otherwise, you can delete the notification now and add one later as needed.

  12. Choose Next.

  13. Under Add a description, enter a name and description for the alarm and choose Next.

  14. Choose Create alarm.

To configure a step scaling policy for your Spot Fleet (console)
  1. Open the Amazon EC2 console at

  2. In the navigation pane, choose Spot Requests.

  3. Select your Spot Fleet request and choose Auto Scaling.

  4. If automatic scaling is not configured, choose Configure.

  5. Use Scale capacity between to set the minimum and maximum capacity for your fleet. Scaling policies do not scale your fleet below the minimum capacity or above the maximum capacity.

  6. For Scaling policies, Policy type, choose Step scaling policy.

  7. Initially, Scaling policies contain step scaling policies named ScaleUp and ScaleDown. You can complete these policies, or choose Remove policy to delete them. You can also choose Add policy.

  8. To define a policy, do the following:

    1. For Policy name, enter a name for the policy.

    2. For Policy trigger, select an existing alarm or choose Create alarm to open the Amazon CloudWatch console and create an alarm.

    3. For Modify capacity, define the amount by which to scale and the lower and upper bound of the step adjustment. You can add or remove a specific number of instances or a percentage of the existing fleet size, or set the fleet to an exact size.

      For example, to create a step scaling policy that increases the capacity of the fleet by 30 percent, choose Add, type 30 in the next field, and then choose percent. By default, the lower bound for an add policy is the alarm threshold and the upper bound is positive (+) infinity. By default, the upper bound for a remove policy is the alarm threshold and the lower bound is negative (-) infinity.

    4. (Optional) To add another step, choose Add step.

    5. For Cooldown period, specify a new value (in seconds) or keep the default.

  9. Choose Save.

To configure step scaling policies for your Spot Fleet using the AWS CLI
  1. Register the Spot Fleet request as a scalable target using the register-scalable-target command.

  2. Create a scaling policy using the put-scaling-policy command.

  3. Create an alarm that triggers the scaling policy using the put-metric-alarm command.