How to use FnImportValue where CDK wants a string or list of string?

[dsilvasc]

Hi,

I'm trying to import a VPC from an existing (non-cdk) stack, following the examples in:
https://gist.github.com/mipearson/aeaf303b0770c25f8b5f6e360594cfbf
#603
#506

I added exports in my existing stack for the VPC ID, CIDR block, availability zones, and subnets.

In order to call VpcNetworkRef.import_, I need to pass it VpcNetworkRefProps, and the builder for the props wants a list of strings for AZs.

None of these type check as arguments to VpcNetworkRefProps.builder().withVpcId(id).withAvailabilityZones:

• new FnImportValue("other-stack-AZs") where other-stack-AZs is a comma-separated export (Fn::Join on the other stack's cloudformation template)
• new FnSplit(",", new FnImportValue("other-stack-AZs"))
• [new FnImportValue("other-stack-AZ1"), new FnImportValue("other-stack-AZ2"), new FnImportValue("other-stack-AZ3")]

How can I import availability zones from another stack?

And more generally, how should we use FnImportValue where CDK wants a string or a list of strings?

[eladb]

CDK tokens, such as FnImportValue can be represented as strings, so you should be able to just do [ new FnImportValue("other-stack-az1").toString(), new FnImportValue("other-stack-az2").toString(), ... ]. Can't say that's super elegant but that should work.

Would that work for your use case?

[dsilvasc]

Thank you -- that would work.

How about in places where CloudFormation accepted Fn::Split but CDK wants a list? For example:

Resources:
  LoadBalancer:
    Type: 'AWS::ElasticLoadBalancingV2::LoadBalancer'
    Properties:
      Subnets: !Split [ ",", !ImportValue shared-infra-PublicSubnetIDs ]
  AutoScalingGroup:
    Type: 'AWS::AutoScaling::AutoScalingGroup'
    Properties:
      VPCZoneIdentifier: !Split [ ",", !ImportValue shared-infra-PrivateSubnetIDs ]

[rix0rrr]

You are being bitten by this issue: #607

For your second problem (can't use a FnSplit() in places where CDK expects a parameter like SomeType[]), I'm afraid we don't really have a solution for that. If you know how many elements will be in the array, you can write it like this:

const parts = new FnSplit(',', new FnImportValue('shared-infra-PublicSubnetIDs);
const array = [
    new SomeType(new FnSelect(0, parts))),
    new SomeType(new FnSelect(1, parts))),
    new SomeType(new FnSelect(2, parts))),
    ...
    ];

But if you don't how many elements there are, unfortunately that won't work.

[eladb]

We don't have support for that, but I wonder if we could somehow support something like FnSplit.toStringArray which will return something that can be assigned to List<String> values. Still not sure how we can achieve something like that (@rix0rrr any idea?)

[rix0rrr]

We could extend Token stringification to have different encodings for different types... but the types would have to be known beforehand which we also don't have.

With the type system that we have, not much we can do I think.

[eladb]

I don't think we need a general-purpose solution, just support for string arrays.