[@aws-cdk/aws-ec2] subnetselect subnets are used for filtering instead of being used directly

[lschierer]

new NetworkLoadBalancer is not detecting public subnets.

Reproduction Steps

import * as r53 from '@aws-cdk/aws-route53';
import * as r53targets from '@aws-cdk/aws-route53-targets';
import * as autoscaling from '@aws-cdk/aws-autoscaling';
import * as elbv2 from '@aws-cdk/aws-elasticloadbalancingv2';
import * as ssm from '@aws-cdk/aws-ssm';

export class SchiererHosts extends core.Construct {

  constructor(scope: core.Construct, id: string) {
    super(scope, id);

    let SchiererVPC = ec2.Vpc.fromVpcAttributes(this, 'SchiererVPC', {
      vpcId: "vpc-0d979df11347689be",
      availabilityZones: [
        "us-east-1a",
        "us-east-1c",
      ],
    });
    let IADazA_Public = new ec2.PublicSubnet(this, "ADazA Public", {
      vpcId: SchiererVPC.vpcId,
      availabilityZone: "us-east-1a",
      mapPublicIpOnLaunch: true,
      cidrBlock: "10.199.41.0/24",
    });
    let IADazC_Public = new ec2.PublicSubnet(this, "ADazC Public", {
      vpcId: SchiererVPC.vpcId,
      availabilityZone: "us-east-1c",
      mapPublicIpOnLaunch: true,
      cidrBlock: "10.199.47.0/24",
    });

    let opusLB = new elbv2.NetworkLoadBalancer(this, "OpusLB", {
      internetFacing: true,
      vpc: SchiererVPC,
      vpcSubnets: {
        subnets: [
          IADazA_Public,
          IADazC_Public,
        ]
      },
    });

  }
}

I also tried to let it pick subnets by itse,f (no vpcSubnets attribute for the NetworkLoadBalancer) and get the same behavior.

What did you expect to happen?

I expect that since I created these brand new subnets using the PublicSubnet constructor, that they would be valid as pubic subnets for NetworkLoadBalancer.

What actually happened?

$ npx cdk synth
There are no 'Public' subnet groups in this VPC. Available types:
Subprocess exited with error 1

Environment

aws/codebuild/standard:4.0 on Linux and Ubuntu 20.04.1 LTS

• CLI Version : cdk version 1.6.6 (for both)
• Framework Version:
• Node.js Version: v12.18.4
• Language (Version): typescript@3.9.7

Other

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This is 🐛 Bug Report

[lschierer]

it looks like it was the error message was just essentially misleading. I had the CIDR ranges on the subnets wrong - they did not match the imported VPC. Once I corrected that, the subnets became public subnets. Mismatched CIDR ranges should tell me that they are mismatched, not produce a subnet that can't be used.

[lschierer]

I take back the last comment, I had removed the elb lines and continued to get the same error. Fixing the CIDR blocks got rid of that, so I thought the problem solved. I commented however before I put the call to new elbv2.NetworkLoadBalancer() back. when I did, the error returned. So something more complicated is going on that I haven't isolated yet.

[rix0rrr]

The issue is probably because subnets is filtering down the subnets it found on the VPC rather than taking the objects you passed. Since the VPC was created without subnets, the empty list filters down to an empty list.

I agree that this is not intuitive.

[lschierer]

so I rather naively thought the ec2.Vpc.fromVpcAttributes was using the attributes listed to identify the VPC to import, and then would discover things about the VPC to fill in the rest of the object's properties. It did not occur to me that anything I did not specify in that call would end up blank. Is that something that I missed in the documentation?

[rix0rrr]

You were probably looking for this: https://docs.aws.amazon.com/cdk/api/latest/docs/aws-ec2-readme.html#importing-an-existing-vpc

[lschierer]

I did start with that function, but I read somewhere in the docs that functions that depend on context do not work well with codepipelines and codebuild yet.

[rix0rrr]

Well. It's not at the location where we'd like it to be vis-a-vis cross-account queries, but as long as you stay within an account (or have access to the target account's credentials), you can do a cdk synth and commit cdk.context.json and everything will work as intended.

[rix0rrr]

Probably relevant for #5927

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