Tokens

Tokens represent values that are unknown until Terraform applies your configuration. For example, names of cloud resources are only assigned upon creation.

Some attributes specified using CDK for Terraform (CDKTF) may not directly map to the values required for Terraform configurations. You can use Tokens to cast these attributes to the correct Terraform language syntax.

Use Tokens

You may need to use Tokens for:

Module outputs for boolean, string, lists, maps, and other complex types.
Resource attributes (such as id).
Terraform outputs based on resource attributes.
Using Terraforms null type.

Example

An 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, you can 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.

import { TerraformStack, TerraformVariable, Token } from "cdktf";
import { Construct } from "constructs";
import { Vpc } from "./.gen/modules/terraform-aws-modules/aws/vpc";
import { Eks } from "./.gen/modules/terraform-aws-modules/aws/eks";

export class TokensStack extends TerraformStack {
  constructor(scope: Construct, id: string, vpcName: string) {
    super(scope, id);

    const logRetention = new TerraformVariable(this, "logRetentionInDays", {
      type: "number",
    });

    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",
      subnetIds: Token.asList(vpc.publicSubnetsOutput),
      cloudwatchLogGroupRetentionInDays: logRetention.numberValue,
    });
  }
}

        TerraformVariable logRetention = new TerraformVariable(this, "logRetentionInDays", TerraformVariableConfig.builder()
                .type("number")
                .build()
        );

        Vpc vpc = new Vpc(this, "vpcName", VpcConfig.builder()
                .name("vpcName")
                .publicSubnets(Arrays.asList("10.0.1.0/24", "10.0.2.0/24"))
                .build()
        );

        new Eks(this, "EksModules", EksConfig.builder()
                .vpcId(vpc.getVpcIdOutput())
                .clusterName("my-kubernetes-cluster")
                .subnetIds(Token.asList(vpc.getPublicSubnetsOutput()))
                .cloudwatchLogGroupRetentionInDays(logRetention.getNumberValue())
                .build()
        );

        log_retention = TerraformVariable(self, "logRetentionInDays",
            type = "number"
        )

        vpc = Vpc(self, "vpcName",
            name = "vpcName",
            public_subnets = ["10.0.1.0/24", "10.0.2.0/24"]
        )

        Eks(self, "EksModules",
            vpc_id = vpc.vpc_id_output,
            cluster_name = "my-kubernetes-cluster",
            subnet_ids = Token.as_list(vpc.public_subnets_output),
            cloudwatch_log_group_retention_in_days = log_retention.number_value
        )

            // Add this to your project's .csproj file:
            // <ItemGroup>
            //     <ProjectReference Include=".gen\vpc\vpc.csproj" />
            // </ItemGroup>
            // <ItemGroup>
            //     <ProjectReference Include=".gen\eks\eks.csproj" />
            // </ItemGroup>
            TerraformVariable cloudwatchLogGroupRetentionInDays = new TerraformVariable(this, "cloudwatchLogGroupRetentionInDays", new TerraformVariableConfig
            {
                Type = "number"
            });

            Vpc vpc = new Vpc(this, "vpc", new VpcConfig
            {
                Name = "vpc",
                PublicSubnets = new string[] { "10.0.1.0/24", "10.0.2.0/24" }
            });

            new Eks(this, "eks", new EksConfig
            {
                ClusterName = "my-kubernetes-cluster",
                SubnetIds = Token.AsList(vpc.PublicSubnetsOutput),
                VpcId = vpc.VpcIdOutput,
                CloudwatchLogGroupRetentionInDays = cloudwatchLogGroupRetentionInDays.NumberValue
            });

import (
    "github.com/aws/constructs-go/constructs/v10"
    "github.com/aws/jsii-runtime-go"
    "github.com/hashicorp/terraform-cdk-go/cdktf"
    "github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/terraform-aws-modules/aws/eks"
    "github.com/hashicorp/terraform-cdk/examples/go/documentation/generated/terraform-aws-modules/aws/vpc"
)

func NewTokensStack(scope constructs.Construct, name string, vpcName string) cdktf.TerraformStack {
    stack := cdktf.NewTerraformStack(scope, &name)

    logRetention := cdktf.NewTerraformVariable(stack, jsii.String("logRetentionInDays"), &cdktf.TerraformVariableConfig{
        Type: jsii.String("number"),
    })

    vpc := vpc.NewVpc(stack, &vpcName, &vpc.VpcConfig{
        Name:          &vpcName,
        PublicSubnets: &[]*string{jsii.String("10.0.1.0/24"), jsii.String("10.0.2.0/24")},
    })

    eks.NewEks(stack, jsii.String("EksModule"), &eks.EksConfig{
        ClusterName:                       jsii.String("my-kubernetes-cluster"),
        SubnetIds:                         cdktf.Token_AsList(vpc.PublicSubnetsOutput(), nil),
        CloudwatchLogGroupRetentionInDays: logRetention.NumberValue(),
    })

    return stack
}

Initially, CDKTF will resolve Token.asList(vpc.publicSubnetsOutput) to ["#{TOKEN[TOKEN.9]}"] and logRetention.numberValue to a big negative number like -123828381238238. Later in synthesis, CDKTF will resolve the token to ${module.<module id>.public_subnets} and ${var.logRetentionInDays}.

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

Refer to the AWS CDK documentation for more detailed information about tokens.

Using Terraform's `null` value

Some edge cases require passing the Terraform null value to for example an attribute of a resource.

import { Token } from "cdktf";
Token.nullValue();

Tokens

Use Tokens

Example

Using Terraform's null value

Using Terraform's `null` value