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Tokens

Use tokens to enable the CDK for Terraform to resolve programming language types to Terraform language syntax, including:

  • Module outputs
  • Resource attributes (such as id)
  • Terraform outputs based on resource attributes

A Terraform module defines multiple resources intended to be used together. Module output values return results to the calling module, which it can then use to populate arguments elsewhere. The CDK for Terraform enables the use of interpolated module outputs as inputs to other modules or resources with an output get method for each output.

In TypeScript, the module output with the AWS VPC identifier from the vpc module gets passed to an AWS EKS cluster by using vpcIdOutput.

const vpc = new Vpc(this, 'my-vpc', {
  name: vpcName
});

new Eks(this, 'EksModule', {
  clusterName: 'my-kubernetes-cluster',
  vpcId: vpc.vpcIdOutput
});

The <output name>Output synthesizes to ${module.<module id>.<output name>}.

{
  "module": {
    "helloterraEksModule5DDB67AE": {
      "cluster_name": "my-kubernetes-cluster",
      "vpc_id": "${module.helloterraMyVpc62D94C17.vpc_id}"
    }
  }
}

CDK for Terraform maps the language types to Terraform 0.12's rich types, such as lists and maps. As a result, some attributes specified using the CDK for Terraform may not map to the string value of <output name>Output. To correct this, use Tokens to cast it to the correct attribute type. Tokens represent values that can only be resolved at a later time in the lifecycle of synthesis.

For example, the EKS module requires a list of subnet ids in order to create a cluster. The VPC module outputs a list of subnets. To pass the subnet id list to the EKS module, we use publicSubnetsOutput to retrieve the list from the VPC. However, the subnets attribute requires a list of strings. Use Token.asList(vpc.publicSubnetsOutput) to cast the interpolated module output as a list of strings.

const vpc = new Vpc(this, vpcName, {
  name: vpcName,
  publicSubnets: ['10.0.1.0/24', '10.0.2.0/24']
});

new Eks(this, 'EksModule', {
  clusterName: 'my-kubernetes-cluster',
  subnets: Token.asList(vpc.publicSubnetsOutput)
});

Initially, the CDK for Terraform will resolve Token.asList(vpc.publicSubnetsOutput) to ["#{TOKEN[TOKEN.9]}"]. Later in synthesis, the CDK for Terraform will resolve the token to ${module.<module id>.public_subnets}.

{
  "module": {
    "helloterraEksModule5DDB67AE": {
      "cluster_name": "my-kubernetes-cluster",
      "subnets": "${module.helloterraMyVpc62D94C17.public_subnets}"
    }
  }
}

Tokens can be used for interpolating module outputs for boolean, string, lists, maps, and other complex type attributes. For more information on Tokens, see AWS CDK documentation.