aws_cdk.aws_ec2.Vpc: max_a_zs ignored?

[dhx-mike-palandra]

• 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?
  For the code below, it seems as if the synthesized template includes only 4 subnets (2 public and 2 private) even if max_a_zs value and subnet configuration would allow for more in the affected region.

If the current behavior is a 🪲bug🪲: Please provide the steps to reproduce
Consider this code:

import sys
from aws_cdk import core
from aws_cdk import aws_ec2 as ec2

app = core.App()

env = core.Environment(region = 'us-east-2')
stack = core.Stack(app, 'vpc-test-cdk036', env = env)
vpc = ec2.Vpc(stack, 'vpc', cidr='10.173.128.0/17', max_a_zs=3,
    subnet_configuration = [
        ec2.SubnetConfiguration(name='pub', subnetType=ec2.SubnetType.PUBLIC),
        ec2.SubnetConfiguration(name='prv', subnetType=ec2.SubnetType.PRIVATE),
    ]
)

print(f'Region: {stack.region}', file=sys.stderr)
print(f'Availability Zones: {stack.availability_zones}', file=sys.stderr)
for subnet_type in ('public', 'private'):
    subnets_attr = f'{subnet_type}_subnets'
    print(f'{subnet_type}: {len(getattr(vpc, subnets_attr))}', file=sys.stderr)

app.synth()

Then, run the CLI like so:
$ cdk synth --json | jq '.Resources | to_entries | [.[] | select(.value.Type == "AWS::EC2::Subnet")] | length'

The jq filter simply counts subnet resources declared in the synthesized template.

• What is the expected behavior (or behavior of feature suggested)?
  Six subnets should have been created. I can't seem to create more than 4. Note that us-east-2 has at least 3 AZs. Also, I notice that no context values were saved by the CLI after a synth command.

I reviewed CHANGELOG.md and commits on master since v0.36.0 but didn't notice anything relevant. I feel like I may have missed something here (sincerest apologies if so), as this kind of issue seems fairly easy to encounter, so I'd be surprised somewhat if indeed this is a bug and I am the first to report it.

• What is the motivation / use case for changing the behavior or adding this feature?
  I'd like to create more than 4 subnets and preserve my existing VPC stacks having more than 4 subnets. Unless I misunderstood something, I'm pretty sure this is a regression because I don't recall having such a problem in v0.34.0. When I run cdk diff from v0.36.0 against similar VPC stacks created by v0.34.0, results indicate a reduction to 4 subnets and change of the remaining subnets to a reduced CIDR prefix (quite disruptive).

• Please tell us about your environment:

  • CDK CLI Version: 0.36.0 (build 6d38487)
  • Module Version: 0.36.0
  • OS: cpe:/o:fedoraproject:fedora:30
  • Language: Python

• 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)

[mhuebner]

Have a look at #3083

[dhx-mike-palandra]

Thank you @mhuebner. This worked as I expected when I replaced:

env = core.Environment(region = 'us-east-2')

with:

import os
env = core.Environment(
        account = os.environ.get('CDK_DEFAULT_ACCOUNT'),
        region = 'us-east-2'
)