AWS ECS service and ELBv2 in separate stacks

Question

It is possible to have a CF stack for an ECS service (e.g. blue/green) and separate stack for the front ELBv2? I've tried having the ELB TargetGroup in the ECS stack and the load balancer and listener in another stack, but the Cloud Formation reports and error when building the target group

"The target group with targetGroupArn xxxxxx does not have an associated load balancer."

Yet there is nothing in the target group resource that defines any load balancer for it to depend on:

 ELB2TargetGroup:
    Type: AWS::ElasticLoadBalancingV2::TargetGroup
    Properties:
      HealthCheckIntervalSeconds: 60
      UnhealthyThresholdCount: 10
      HealthCheckPath: /
      Port:     !Ref ALBPort
      Protocol: !Ref ALBProtocol
      VpcId:
        Fn::ImportValue: !Sub "${ECSClusterStack}-ClusterVPC"

Answer 1

I hope you have resolved this issue already. But, just in case someone else comes here looking for the same issue.

Essentially, you have done nothing wrong. Just that, you need a Listener Rule to associate the Target Group with the Load Balancer Listener.

Here's an example TargetGroup resource:

...
ExampleTG:
  Type: AWS::ElasticLoadBalancingV2::TargetGroup
  Properties:
    HealthCheckIntervalSeconds: 30
    HealthCheckProtocol: HTTP
    HealthCheckTimeoutSeconds: 5
    HealthyThresholdCount: 2
    HealthCheckPath: /healthcheck
    HealthCheckPort: 80
    Matcher:
      HttpCode: '200'
    Name: !Join [ "-", [ Example, !Ref SomeParameter] ]
    Port: 80
    Protocol: HTTP
    TargetGroupAttributes:
    - Key: deregistration_delay.timeout_seconds
      Value: '15'
    UnhealthyThresholdCount: 3
    VpcId:
      'Fn::ImportValue': !Sub '${ParentVPCStack}-vpc'

I think you had that already. All you need now is a ListernerRule:

...
ExampleListernerRule:
  Type: AWS::ElasticLoadBalancingV2::ListenerRule
  Properties:
    Actions:
      - Type: forward
        TargetGroupArn: !Ref ExampleTG
    Conditions:
      - Field: host-header
        Values:
        - "example.*"
    ListenerArn:
      'Fn::ImportValue': !Sub '${ParentLBStack}-existing-listener'
    Priority: 1

You can easily import the listener from an existing stack [ELBv2 in your case], which associates the Load Balancer with the Target Group.