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• I'm submitting a ...

  • 🪲 bug report
  • 🚀 feature request
  • 📚 construct library gap
  • ☎️ security issue or vulnerability => Please see policy
  • ❓ support request => Please see note at the top of this template.

• What is the current behavior?
  If the current behavior is a 🪲bug🪲: Please provide the steps to reproduce

Creation of an ALB is failing with an error that it is getting more than one subnet per AZ.

A load balancer cannot be attached to multiple subnets in the same Availability Zone (Service: AmazonElasticLoadBalancingV2; Status Code: 400; Error Code: InvalidConfigurationRequest;

code to reproduce:

      const vpcId = "vpc-xxxxxx";

      const vpc = ec2.Vpc.fromLookup(this, "Vpc", { vpcId: vpcId });

      const alb = new elbv2.ApplicationLoadBalancer(this, id + "LoadBalancer", {
        loadBalancerName: id + 'ALB',
        vpc: vpc,
        internetFacing: false,
        vpcSubnets: {subnetType: ec2.SubnetType.PRIVATE, onePerAz: true}
      });

cdk synth shows a long list of subnets.

console.log(vpc.selectSubnets({subnetType: ec2.SubnetType.PRIVATE, onePerAz: true}).availabilityZones outputs

[
  'us-east-1c', 'us-east-1d',
  'us-east-1d', 'us-east-1d',
  'us-east-1d', 'us-east-1d',
  'us-east-1d', 'us-east-1d',
  'us-east-1d', 'us-east-1d',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e',
  'us-east-1e', 'us-east-1e'
]

Having looked at the implementation, it seems that all private subnets retrieved for the VPC have "Private" returned from subnetName() and the implementation of onePerAz simply filters like this:

subnets = subnets.filter(s => subnetName(s) === subnetName(subnets[0]));

so it returns all the subnets, not just one per AZ.

Selecting by subnetName does not actually seem to use the Name shown in the AWS console.

• What is the expected behavior (or behavior of feature suggested)?
  onePerAz: true should return exactly one subnet per AZ.

• What is the motivation / use case for changing the behavior or adding this feature?
  trying to create an ALB inside an existing VPC

• Please tell us about your environment:

  • CDK CLI Version: 0.36.0 (build 6d38487)
  • Module Version: "@aws-cdk/aws-ec2": "^0.36.0"
  • OS: OSX Mojave
  • Language: TypeScript

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. associated pull-request, stackoverflow, gitter, etc)