Issues attaching ASG to an existing ALB listener

[bolsote]

As part of a larger migration of existing assets into a CDK-managed stack, I have to spin up a new ASG, attached to a load balancer, using the Python flavour of the CDK.

This is the approach I've followed (ignoring the lookup of additional resources, such as the VPC and SGs):

# ASG creation.
asg = aws_autoscaling.AutoScalingGroup(
    self,
    "appGroupDev",
    vpc=vpc,
    role=role,
    instance_type=aws_ec2.InstanceType("t3.micro"),
    machine_image=ami,
    user_data=user_data,
    key_name="app_key",
    block_devices=[
        aws_autoscaling.BlockDevice(
            device_name="/dev/sda1",
            volume=aws_autoscaling.BlockDeviceVolume.ebs(
                volume_size=100,
                delete_on_termination=True,
            ),
        ),
    ],
    desired_capacity=1,
    min_capacity=1,
    max_capacity=3,
)
asg.add_security_group(sg[0])
asg.add_security_group(sg[1])

# Target group creation.
target_group = aws_elasticloadbalancingv2.ApplicationTargetGroup(
    self,
    "appTargetGroup",
    vpc=vpc,
    targets=[asg],  # Registration.
    health_check=aws_elasticloadbalancingv2.HealthCheck(
        path="/healthcheck",
        port="8080",
    ),
    deregistration_delay=Duration.seconds(0),
)

# Listener attachment.
listener = aws_elasticloadbalancingv2.ApplicationListener.from_lookup(
    self,
    "appHttpListener",
    listener_arn=LISTENER_ARN,
)
listener.add_target_groups(
    "appTargetGroupAttachment",
    target_groups=[target_group],
    conditions=[
        aws_elasticloadbalancingv2.ListenerCondition.path_patterns(
            values=["/app*"]
        ),
    ],
    priority=10,
)

Unfortunately, this does not work, and complains on synth:

RuntimeError: Cannot read properties of undefined (reading 'toString')

If I remove the targets=[asg] registration line from the ApplicationTargetGroup() instantiation, and instead do:

asg.attach_to_application_target_group(target_group)

I still get the exact same problem.

I wonder whether mixing new and existing resources is the problem here, and I need a slightly different incantation, I am doing something that's off enough to cause this, or this is a legitimate issue. Could anyone point me in the right direction?

[peterwoodworth]

Either your port or protocol must be defined on your ApplicationTargetGroup, so that the port can be known. The implementation we have is a bit buggy in that we are running this code without first verifying that the port is defined, leading to an unhelpful error message

aws-cdk/packages/aws-cdk-lib/aws-elasticloadbalancingv2/lib/alb/application-target-group.ts

Lines 409 to 410 in 4e0d726

	public registerConnectable(connectable: ec2.IConnectable, portRange?: ec2.Port) {
	portRange = portRange || ec2.Port.tcp(this.defaultPort);

[bolsote]

Thank you very much, that did solve the issue, and a CloudFormation template was successfully generated. The error message was indeed confusing, but the fix makes complete sense.

[github-actions[bot]]

Hello! Reopening this discussion to make it searchable.