feat(elbv2): default config for internal load balancer will fall back to Isolated subnets

[flemjame-at-amazon]

---

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Added a secondary default subnet selection for Load Balancers such that, if I have an isolated VPC like so:

Vpc vpc = new Vpc(construct, "IsolatedVpc", VpcProps.builder()
        .subnetConfiguration(
                Arrays.asList(
                        SubnetConfiguration.builder()
                                .subnetType(SubnetType.ISOLATED)
                                .name("isolated").build()
                ))
        .build());

I don't have to specify the subnets for an internal load balancer. Currently, users must specify their Isolated subnets like so:

NetworkLoadBalancer nlb = new NetworkLoadBalancer(this, "NLB", NetworkLoadBalancerProps.builder()
        .internetFacing(false)
        .vpcSubnets(SubnetSelection.builder()
                .subnetType(SubnetType.ISOLATED)
                .build())
        .vpc(vpc)
        .build());

With this change, if there are no Private subnets and internetFacing is false, then Isolated subnets will be selected, allowing users to code their NLB like so:

NetworkLoadBalancer nlb = new NetworkLoadBalancer(this, "NLB", NetworkLoadBalancerProps.builder()
        .internetFacing(false)
        .vpc(vpc)
        .build());

This doesn't break the default behavior, which places a LB into Private subnets in the event that both Private and Isolated subnets exist. Users who are already specifying Isolated subnets can continue to do so, as I did not change the logic if the user specifies vpcSubnets.

Commit Message

feat(elbv2): default config for internal load balancer will fall back to Isolated subnets

Internal load balancers used to be always created in Private subnets, and hence fail if the VPC does not include Private subnets.

Now using the default VPC "private subnets" fallback mechanism which fixes this case.

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[rix0rrr]

I recently updated Vpc to have some of this behavior already (private => isolated as a fallback).

Can you try the following and see if that behaves the same?

const vpcSubnets = ... (internetFacing ? { subnetType: PUBLIC } : {});

I.e. leave subnetType empty, rely on the Vpc default to try private => isolated => public in that order?

[flemjame-at-amazon]

That just worked, no additional changes to tests needed.

Pull request has been modified.

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Pull request has been modified.

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