Scaling a Cluster
If your cluster was created with the console first-run experience described in Getting Started with Amazon ECS after November 24th, 2015, then the Auto Scaling group associated with the AWS CloudFormation stack created for your cluster can be scaled up or down to add or remove container instances. You can perform this scaling operation from within the Amazon ECS console.
If your cluster was not created with the console first-run experience described in Getting Started with Amazon ECS after November 24th, 2015, then you cannot scale your cluster from the Amazon ECS console. However, you can still modify existing Auto Scaling groups associated with your cluster in the Auto Scaling console. If you do not have an Auto Scaling group associated with your cluster, you can create one from an existing container instance. For more information, see Creating an Auto Scaling Group Using an EC2 Instance in the Auto Scaling User Guide. You can also manually launch or terminate container instances from the Amazon EC2 console; for more information see Launching an Amazon ECS Container Instance.
To scale a cluster
Open the Amazon ECS console at https://console.aws.amazon.com/ecs/.
From the navigation bar, choose the region that your cluster exists in.
In the navigation pane, choose Clusters.
Choose the cluster that you want to scale.
On the Cluster :
namepage, choose the ECS Instances tab.
If a Scale ECS Instances button appears, then you can scale your cluster in the next step. If not, you must manually adjust your Auto Scaling group to scale up or down your instances, or you can manually launch or terminate your container instances in the Amazon EC2 console.
Choose Scale ECS Instances.
In the Desired number of instances field, enter the number of instances you wish to scale your cluster to and choose Scale.
If you reduce the number of container instances in your cluster, any tasks that are running on terminated instances are stopped.