Control the instances Amazon ECS terminates

Important

You must turn on Auto Scaling instance scale-in protection on the Auto Scaling group to use the managed termination protection feature of cluster auto scaling.

Managed termination protection allows cluster auto scaling to control which instances are terminated. When you used managed termination protection, Amazon ECS only terminates EC2 instances that don't have any running Amazon ECS tasks. Tasks that are run by a service that uses the DAEMON scheduling strategy are ignored and an instance can be terminated by cluster auto scaling even when the instance is running these tasks. This is because all of the instances in the cluster are running these tasks.

Amazon ECS first turns on the instance scale-in protection option for the EC2 instances in the Auto Scaling group. Then, Amazon ECS places the tasks on the instances. When all non-daemon tasks are stopped on an instance, Amazon ECS initiates the scale-in process and turns off scale-in protection for the EC2 instance. The Auto Scaling group can then terminate the instance.

Auto Scaling instance scale-in protection controls which EC2 instances can be terminated by Auto Scaling. Instances with the scale-in feature turned on can't be terminated during the scale-in process. For more information about Auto Scaling instance scale-in protection, see Using instance scale-in protection in the Amazon EC2 Auto Scaling User Guide.

You can set the targetCapacity percentage so that you have spare capacity. This helps future tasks launch more quickly because the Auto Scaling group does not have to launch more instances. Amazon ECS uses the target capacity value to manage the CloudWatch metric that the service creates. Amazon ECS manages the CloudWatch metric. The Auto Scaling group is treated as a steady state so that no scaling action is required. The values can be from 0-100%. For example, to configure Amazon ECS to keep 10% free capacity on top of that used by Amazon ECS tasks, set the target capacity value to 90%. Consider the following when setting the targetCapacity value on a capacity provider.

• A targetCapacity value of less than 100% represents the amount of free capacity (Amazon EC2 instances) that need to be present in the cluster. Free capacity means that there are no running tasks.

• Placement constraints such as Availability Zones, without additional binpack forces Amazon ECS to eventually run one task for each instance, which might not be the desired behavior.

You must turn on Auto Scaling instance scale-in protection on the Auto Scaling group to use managed termination protection. If you don't turn on scale-in protection, then turning on managed termination protection can lead to undesirable behavior. For example, you may have instances stuck in draining state. For more information, see Using instance scale-in protection in the Amazon EC2 Auto Scaling User Guide.

When you use termination protection with a capacity provider, don't perform any manual actions, like detaching the instance, on the Auto Scaling group associated with the capacity provider. Manual actions can break the scale-in operation of the capacity provider. If you detach an instance from the Auto Scaling group, you need to also deregister the detached instance from the Amazon ECS cluster.