Question

I have two (and several other) stacks which create a cyclic dependency:

app.py

app = cdk.App()
vpc_stack = RcsVpcStack(app, "RcsVPCStack")
efs_stack = RcsEfsStack(app, "RcsEfsStack", vpc_stack=vpc_stack)
ecs_stack = RcsECSStack(app, "RcsECSStack", vpc_stack=vpc_stack, fileSystem=efs_stack.efs_filesystem)

elbv2_stack = RcsELBv2Stack(app, "RcsELBv2Stack", 
                            vpc_stack=vpc_stack,  
                            cert_arn=CERT_ARN
                            )
ecs_service_stack = RcsECSServiceStack(app, "RcsECSServiceStack", 
                   vpc_stack=vpc_stack, 
                   ecs_stack=ecs_stack,
                   elbv2_stack=elbv2_stack
                   )

Elbv2Stack
defines an ApplicationLoadbalancer with listener

  self.domain_cert = certm.Certificate.from_certificate_arn(
      self,
      "rcs-domain-cert",
      cert_arn
  )
  self.elbv2_loadbalancer = elbv2.ApplicationLoadBalancer(
      self,
      "rcs-elastic-loadbalancer",
      load_balancer_name="rcs-elastic-loadbalancer",
      vpc=vpc_stack.vpc,
      vpc_subnets=ec2.SubnetSelection(
          subnets=vpc_stack.vpc.public_subnets
      ),
      idle_timeout=Duration.seconds(300),
      internet_facing=True
  )
  self.elbv2_loadbalancer_listener = self.elbv2_loadbalancer.add_listener(
      "rcs-elastic-loadbalancer-listener",
      port=443,
      protocol=elbv2.ApplicationProtocol.HTTPS,
      certificates=[elbv2.ListenerCertificate.from_certificate_manager(self.domain_cert)]
  )
  self.elbv2_loadbalancer_listener.add_targets(
      "rcs-default-target-group",
      port=80,
      protocol=elbv2.ApplicationProtocol.HTTP
  )

EcsStack
defines an ECS Container service

  # ... service definitions 

  elbv2_stack.elbv2_loadbalancer_listener.add_targets(
      "rcs-api-backend-loadbalancer-target",
      port=80,
      targets=[
          ecs_service_stack.ecs_api_backend.load_balancer_target(
              container_name="testService",
              container_port=someContainerPort
          )
      ]
  )
  self.test_target_group = elbv2.ApplicationTargetGroup(
      self,
      "rcs-api-backend-target-group",
      vpc=vpc_stack.vpc,
      port=80,
      protocol=elbv2.ApplicationProtocol.HTTP,
      health_check=elbv2.HealthCheck(
          healthy_http_codes="200-299",
          interval=Duration.seconds(20),
          path="/health-check",
          protocol=elbv2.Protocol.HTTP,
          timeout=Duration.seconds(5),
          healthy_threshold_count=2
      )
  )
  elbv2_stack.elbv2_loadbalancer_listener.add_action(
      "rcs-api-backend-elbv2-action",
      action=elbv2.ListenerAction.forward(
          target_groups=[self.test_target_group],
      ),
      priority=2,
      conditions=[
          elbv2.ListenerCondition.path_patterns(["*"])
      ]
  )

The Error:

RuntimeError: Error:  RcsECSServiceStack  depends on  RcsELBv2Stack  
({RcsECSServiceStack/rcs-api-backend/Service}.addDependency({RcsELBv2Stack/rcs-elastic-loadbalancer/rcs-elastic-loadbalancer-listener/Resource}), 
{RcsECSServiceStack/rcs-api-backend/Service}.addDependency({RcsELBv2Stack/rcs-elastic-loadbalancer/rcs-elastic-loadbalancer-listener/rcs-default-target-groupGroup/Resource}), 
{RcsECSServiceStack/rcs-api-backend/Service}.addDependency({RcsELBv2Stack/rcs-elastic-loadbalancer/rcs-elastic-loadbalancer-listener/rcs-api-backend-loadbalancer-targetGroup/Resource}), 
{RcsECSServiceStack/rcs-api-backend/Service}.addDependency({RcsELBv2Stack/rcs-elastic-loadbalancer/rcs-elastic-loadbalancer-listener/rcs-api-backend-elbv2-actionRule/Resource})). Adding this dependency (RcsELBv2Stack -> RcsECSServiceStack/rcs-api-backend-target-group/Resource.Ref) would create a cyclic reference.

To have an overview, it would be great to define the container service with its target group in the dedicated stack, but I get a cyclic dependency if I do that.
Is it a problem with the action added?
Is there a possible solution to this?

Answer 1

From our experience, we had to separate how we create an ECS service and a Load Balancer. We ended up having an ECS Service construct that can be deployed even without a load balancer and target group attached to it.

The ECS service construct we made have optional parameters to put in the load balancer ARN and the load balancer HTTPS listener ARN. We then use LookUps to reference those ALB and Listeners.

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