Autoscaling for ECS EC2 standalone tasks - amazon-web-services

Question

I am using CDK to run standalone containerized ECS tasks on EC2.
I'm not using ECS service, because my task should be triggered using a request (in this a lambda), so they are not long running (or aways on). I use the following code, which works for one instance of the task:
this.autoScalingGroup = new autoscaling.AutoScalingGroup(this, 'ASG', {
vpc: this.vpc,
instanceType: ec2.InstanceType.of(
ec2.InstanceClass.C5,
ec2.InstanceSize.XLARGE,
),
machineImage: ecs.EcsOptimizedImage.amazonLinux2(),
minCapacity: 1,
maxCapacity: 10,
updatePolicy: UpdatePolicy.rollingUpdate(),
});
const capacityProvider = new ecs.AsgCapacityProvider(
this,
'AsgCapacityProvider',
{
autoScalingGroup: this.autoScalingGroup,
},
);
this.cluster.addAsgCapacityProvider(capacityProvider);
The problem is that I cannot run concurrent instances. So, it doesn't scale up to more instances. It only runs one instance (my container takes up ~80% of the EC2 instance resources). How would I trigger autoscaling such that when multiple tasks are run more instances get added?
p.s. obviosuly if I set minCapacity to x, x number of concurrent tasks can run, but I'd like to only keep 1 instance up and scale up and down.

Answer 1

The CDK-created capacity provider will auto-scale the instances based on standalone task activity. I suspect scaling is not working for you because your run-task invocations are missing a capacity provider strategy that tells ECS what capacity provider to use. There are a couple of ways to fix this:
Option 1: Set a strategy in the run-task call:
aws ecs run-task \
--cluster <cluster-arn> \
--task-definition <task-definition-name> \
--capacity-provider-strategy capacityProvider=<capacity-provider-name>,weight=1,base=1 \
--count 3 \
Option 2: Set the cluster's *default* capacity provider strategy
The CDK's L2 Cluster construct does not set a default capacity provider strategy on your cluster. See this github issue for context and workarounds. The default can also be set in the console.