[ec2-vpc]: SubnetSelection with AZs with vpc contruct in same application

[annguyen36]

Describe the issue

Hello team,

We identified an issue: the .selectSubnets() method doesn't function with availabilityZones when the VPC is within the same application, example snippet:

  const supportedAZs = ['us-east-1d', 'us-east-1a', 'us-east-1c'];
  const selectedSubnets = vpc.selectSubnets({
          availabilityZones: supportedAZs,
          subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS
      });

Upon verification, it's apparent that the AZ value acts as the TOKEN placeholder during the stack's synthesis, leading the above selectedSubnets to return empty. Notably, this operates correctly if the VPC is imported from an existing one.

Should this be intended behavior, might we request an update to the documentation, specifically regarding the availabilityZones parameter within the SubnetSelection interface, to prevent future confusion?

Links

e

[pahud]

const vpc = new ec2.Vpc(this, 'Vpc', { maxAzs: 3 });
const supportedAZs = ['us-east-1d', 'us-east-1a', 'us-east-1c'];
const selectedSubnets = vpc.selectSubnets({
		availabilityZones: supportedAZs,
		subnetType: ec2.SubnetType.PRIVATE_WITH_EGRESS
});

new CfnOutput(this, 'selectedSubnets', { value: selectedSubnets.subnetIds.join(',')})

If you create a new VPC in the app, you will not now the selected subnetIds until deployment so you will only get the tokens.

Can you specify which document statement should be improved?

[annguyen36]

Hi pahud,
Thanks for confirmation.
I would suggest add this in the availibilityZones prop description in this doc https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2.SubnetSelection.html#availabilityzones