why NetworkLoadBalancer.fromLookup returning my-load-balancer

Question

I am trying to get a network loadbalancer using tags, but not sure why I am getting my-load-balancer-1234567890.us-west-2.elb.amazonaws.com
See below code

```
const nlb = this.getNLBByTag(props);
this.nlbDnsName = nlb.loadBalancerDnsName;

private getNLBByTag(props: ABCDomainVPCStackProps): INetworkLoadBalancer{
        const nlbOptions: NetworkLoadBalancerLookupOptions = {
            loadBalancerTags: {
              component: "apigw",
              application: "abc",
              env: "dev"
            },
        };
        
        return NetworkLoadBalancer.fromLookup(this, `abc-${props.stage}-nlb`, nlbOptions);
    }
```

Basically it is returning 3 results.
First one is "my-load-balancer-1234567890.us-west-2.elb.amazonaws.com"
2nd and 3rd are same which I am looking for, but not sure why it is returning 2nd and 3rd as the same.
I have checked only 1 nlb has above tags.

Harsha · Answer

Hello,

From the description, I understand that you are using "NetworkLoadBalancer.fromLookup" in CDK and you are getting the responses of below format for the ".loadBalancerDnsName" method on the obtained object.

my-load-balancer-1234567890.us-west-2.elb.amazonaws.com  
-XXXXXX.elb..amazonaws.com

I would like to inform you that I was able to replicate the same in my test environment. Kindly note that this behaviour is expected and occurs because on the first "cdk deploy/synth" if the "cdk.context.json" file is empty and/or does not contain the load-balancer details.

For all the subsequent "cdk deploy", the correct value was obtained as the "cdk.context.json" file would be populated with the required details. You can learn more about "cdk context" at doc[1].

Hence, you can perform "cdk deploy/synth" and check the "cdk.context.json" file for the load-balancer details as shown below:

```
  "load-balancer:account=XXXXXX: {
    "loadBalancerArn": "arn:aws:elasticloadbalancing:XXXXXX",
    "loadBalancerCanonicalHostedZoneId": "XXXXXX",
    "loadBalancerDnsName": "-XXXXXX.elb..amazonaws.com",
    "vpcId": "vpc-XXXXX",
    "securityGroupIds": [],
    "ipAddressType": "ipv4"
  }
```

[1] https://docs.aws.amazon.com/cdk/v2/guide/context.html

why NetworkLoadBalancer.fromLookup returning my-load-balancer

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