What is a Token in the AWS Cloud Development Kit (CDK)?

The AWS Cloud Development Kit (CDK) is a software development framework for defining cloud infrastructure in code. One important concept in the CDK is the concept of a token. In this blog post, we'll explore what tokens are in the CDK and provide an example of how to use them.

A token in the CDK is a placeholder for a value that will be determined at deployment time. Tokens are used to represent values that are not known at the time of writing the CDK code but are instead determined by the CDK framework during deployment. These can be used to reference resources or parameters that haven't been created yet.

There are two types of tokens in the CDK: intrinsic functions and construct tokens.

• Intrinsic functions are special functions that are part of the CDK framework and return tokens. For example, Fn.importValue() is an intrinsic function that returns a token representing the value of output from another CloudFormation stack.

const importedValue = cdk.Fn.importValue('ImportedValue');

• Construct tokens are tokens that are returned by constructs (classes that represent cloud resources in the CDK) that can be used to reference other resources within the same stack.

class MyStack extends cdk.Stack {

  public readonly output: cdk.CfnOutput;

  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);
    // ... some other code
    this.output = new cdk.CfnOutput(this, 'output', { value:  
    'output' });
  }
}

const myStack = new MyStack(app, 'MyStack');

myStack.output.export();

In both of the above examples, importedValue and myStack.output variables are tokens that can be used to reference the specific resources or values defined in the CDK.

In this blog post, we've discussed what tokens are in the AWS CDK and provided examples of how to use intrinsic functions and construct tokens. Tokens are an important concept in the CDK as they allow you to reference values that are not known at the time of writing your CDK code but will be determined during deployment. Understanding how to work with tokens is key to effectively using the CDK to define your cloud infrastructure.

Additionally, Tokens in CDK are also used to pass dynamic values between different construct instances. For example, You can use Tokens to pass dynamic values from one construct instance to another. For this purpose, you can use the token.asXxx method.

const vpcId = ec2.Vpc.fromLookup(this, 'VPC', { vpcName: 'MyVPC' }).vpcId;

const subnetId = ec2.Subnet.fromSubnetAttributes(this, 'Subnet', {
  vpcId: vpcId.toString(),
  subnetName: 'MySubnet'
}).subnetId;

Here, the vpcId is a token and is passed to the vpcId property of the fromSubnetAttributes method and it returns the subnetId token.

Also, Tokens can be used to define conditional CloudFormation resources.

const myBucket = new s3.Bucket(this, 'MyBucket', {
    versioned: someValue.asBool()
});

Here, the someValue is a token and asBool() converts it to a boolean value and it can be used as the value of versioned property.

As you can see, Tokens play a crucial role in the AWS CDK and they allow you to pass dynamic values, reference resources, and make CloudFormation resources conditional. Understanding how to use tokens can help you to take full advantage of the CDK's capabilities and to define cloud infrastructure that is adaptable and flexible.

Another common use case of Tokens is when you need to reference resources that are not created by your CDK code, but already exist in your AWS account. For example, you might want to reference an existing VPC or an existing SNS topic in your CDK code. In these cases, you can use the cdk.Fn.importValue method to import the value of the resource and store it in a token.

const existingVpcId = cdk.Fn.importValue('existing-vpc-id');

const myVpc = ec2.Vpc.fromVpcAttributes(this, 'MyVPC', {
    vpcId: existingVpcId
});

Here, the existing-vpc-id are the physical name of an output exported by another CloudFormation stack and we can use it to import the value of the VPC id and use it to reference the existing VPC in our CDK code.

Another important aspect of tokens is that they can be used together with cdk.CfnOutput and cdk.CfnParameter classes to export and import values between different CDK Stacks or even between different CloudFormation Stacks. By using tokens in these classes, you can pass dynamic values between different CDK Stacks or import values from other CloudFormation stacks to your CDK code.

In conclusion, tokens are an essential concept in the AWS CDK. They allow you to pass dynamic values, reference resources and make CloudFormation resources conditional, import values from other resources, and make your CloudFormation code more adaptable and flexible. Understanding how to use tokens effectively can help you to take full advantage of the CDK's capabilities and to define cloud infrastructure that meets your specific needs.