AWS CDK: How should I attach an ECS service to an existing load balancer?

Question

I have a pre-existing load balancer on AWS, and I would like to spin a stack with an ECS service that is targeted by that load balancer.

I found this solution that suggests using an ApplicationLoadBalancedFargateService; however, that construct only works creating a new listener, while I want to use the existing listener (or should I want a new listener?).

Reading the source code here and here, I tried to use the same functions for the looked-up listener, but apparently cdk "Cannot add default Target Groups to imported ApplicationListener" (from cdk errors).

My best attempt at the moment:

// cdk-stack.js inside the constructor
let service = new ecs.FargateService(this, id + '-service', {
  serviceName: id + '-service',
  cluster: cluster,
  taskDefinition: taskDefinition,
  securityGroups: [thisSecurityGroup],
});

const loadBalancer = elbv2.ApplicationLoadBalancer.fromLookup(this, 'bip-lb', { loadBalancerArn: loadBalancerArn });

const listener = elbv2.ApplicationListener.fromLookup(this, 'https-listener', {
  loadBalancerArn: loadBalancer.loadBalancerArn,
  listenerProtocol: elbv2.ApplicationProtocol.HTTPS,
  listenerPort: 443,
});

const targetGroup = new elbv2.ApplicationTargetGroup(this, id + '-target-group', {
  targetGroupName: id + '-target-group',
});

listener.addTargetGroups(id + '-target-group', {targetGroups: [targetGroup]});

targetGroup.addTarget(service);

Answer 1

Add a priority prop to listener.addTargetGroups. Under the hood, the method creates a new ApplicationListenerRule construct. The rule requires an explicit priority.

For this reason, the CDK throws the observed error when addTargetGroups does not have a priority prop defined..