How to get the URL of the ALB after creating in CDK? - amazon-elb

Question

I'm creating an Application Load Balancer using the AWS CDK v2.
This is my code:
const lb = new elb.ApplicationLoadBalancer(this, 'LB', {
vpc: ec2.Vpc.fromLookup(this, 'vpc-lookup', {
isDefault: true
}),
internetFacing: true
});
const listener = lb.addListener('Listener', {
port: 80,
});
My question is how do I get the URL (DNS name) of the load balancer? I need it in the CDK after to update something

Answer 1

TL;DR The name's actual value is resolved at deploy-time. At synth-time, you can pass loadBalancerDnsName to other constructs and CDK will create the necessary references.
Resource identifiers like DNS addresses are generally known only only at deploy-time. The CDK uses Tokens to "represent values that can only be resolved at a later time in the lifecycle of an app" . ApplicationLoadBalancer's loadBalancerDnsName: string property is one of those properties whose value resolves to a string Token placeholder
at synth-time and an actual value at deploy-time.
Here's an example of passing the loadBalancerDnsName between constructs:
export class AlbStack extends cdk.Stack {
constructor(scope: Construct, id: string, props: cdk.StackProps) {
super(scope, id, props);
const alb = new elb.ApplicationLoadBalancer(this, 'MyALB', {
vpc: ec2.Vpc.fromLookup(this, 'DefaultVpc', { isDefault: true }),
});
// WON'T WORK: at synth-time, the name attribute returns a Token, not the expected DNS name
console.log(alb.loadBalancerDnsName); // ${Token[TOKEN.220]}
// WILL WORK - CDK will wire up the token in CloudFormation as
new ssm.StringParameter(this, 'MyAlbDns', {
stringValue: alb.loadBalancerDnsName,
});
}
}
The CDK's CloudFormation template output has Fn::GetAtt placeholder for the DNS name that resolves at deploy-time:
// CDK CloudFormation stack template
// Resources section
"MyAlbDnsFD44EB27": {
"Type": "AWS::SSM::Parameter",
"Properties": {
"Type": "String",
"Value": { "Fn::GetAtt": [ "MyALB911A8556", "DNSName" ] } // this will resolve to the string at deploy
},
"Metadata": {
"aws:cdk:path": "TsCdkPlaygroundAlbStack/MyAlbDns/Resource"
}
},