Understanding CDK and CloudFormation

Oct 13, 2025 by wbrum.org 115 views

Introduction

When working with AWS Cloud Development Kit (CDK), you may encounter situations where you need to reference the output variable value of an existing CloudFormation stack in your current CDK stack. This can be a challenging task, especially if you're new to CDK. In this article, we'll explore the different ways to achieve this and provide a step-by-step guide on how to get the output variable value of an existing CloudFormation stack in your CDK stack.

Understanding CDK and CloudFormation

Before we dive into the solution, let's quickly understand the basics of CDK and CloudFormation.

CDK: CDK is an open-source framework provided by AWS that allows developers to define cloud infrastructure in code. It provides a set of high-level APIs that make it easy to create cloud infrastructure, such as AWS Lambda functions, Amazon S3 buckets, and more.
CloudFormation: CloudFormation is a service provided by AWS that allows you to create and manage cloud infrastructure in the form of templates. These templates can be used to create and manage resources such as EC2 instances, RDS databases, and more.

Getting the Output Variable Value of an Existing CloudFormation Stack

To get the output variable value of an existing CloudFormation stack in your CDK stack, you can use the core.Fn.import_values method. This method allows you to import the output values of an existing CloudFormation stack into your CDK stack.

Here's an example of how to use the core.Fn.import_values method:

import * as cdk from 'aws-cdk-lib';
import * as iam from 'aws-cdk-lib/aws-iam';
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cloudformation from 'aws-cdk-lib/aws-cloudformation';
export class MyStack extends cdk.Stack {
constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);
// Import the output values of the existing CloudFormation stack
const stackName = &#39;my-existing-stack&#39;;
const outputName = &#39;MyOutput&#39;;
const outputValue = cloudformation.CloudFormationStack.import_values(this, stackName, outputName);

// Use the output value in your CDK stack
const myOutput = outputValue.toString();
console.log(myOutput);

}
}

In this example, we're importing the output value of the MyOutput output variable from the my-existing-stack CloudFormation stack. We're then using the output value in our CDK stack.

Using the `core.Fn.get_att` Method

Another way to get the output variable value of an existing CloudFormation stack is by using the core.Fn.get_att method. This method allows you to get the value of an output variable from an existing CloudFormation stack.

Here's an example of how to use the core.Fn.get_att method:

import * as cdk from 'aws-cdk-lib';
import * as iam from 'aws-cdk-lib/aws-iam';
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cloudformation from 'aws-cdk-lib/aws-cloudformation';
export class MyStack extends cdk.Stack {
constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);
// Get the output value of the existing CloudFormation stack
const stackName = &#39;my-existing-stack&#39;;
const outputName = &#39;MyOutput&#39;;
const outputValue = cdk.Fn.get_att(`${stackName}.Outputs.${outputName}`);

// Use the output value in your CDK stack
const myOutput = outputValue.toString();
console.log(myOutput);

}
}

In this example, we're getting the output value of the MyOutput output variable from the my-existing-stack CloudFormation stack using the core.Fn.get_att method. We're then using the output value in our CDK stack.

Conclusion

In this article, we've explored the different ways to get the output variable value of an existing CloudFormation stack in your CDK stack. We've used the core.Fn.import_values method and the core.Fn.get_att method to achieve this. By following the steps outlined in this article, you should be able to get the output variable value of an existing CloudFormation stack in your CDK stack.

Best Practices

Here are some best practices to keep in mind when working with CDK and CloudFormation:

Use the core.Fn.import_values method: This method is the recommended way to import the output values of an existing CloudFormation stack into your CDK stack.
Use the core.Fn.get_att method: This method can be used to get the value of an output variable from an existing CloudFormation stack.
Use the cdk.Stack class: This class provides a set of methods that make it easy to work with CloudFormation stacks in your CDK stack.
Use the cloudformation.CloudFormationStack class: This class provides a set of methods that make it easy to work with CloudFormation stacks in your CDK stack.

By following these best practices, you should be able to work with CDK and CloudFormation effectively and efficiently.

Common Issues

Here are some common issues that you may encounter when working with CDK and CloudFormation:

Error: core.Fn.get_att is not a function: This error occurs when you're trying to use the core.Fn.get_att method without importing the aws-cdk-lib module.
Error: cloudformation.CloudFormationStack is not a function: This error occurs when you're trying to use the cloudformation.CloudFormationStack class without importing the aws-cdk-lib module.
Error: cdk.Stack is not a function: This error occurs when you're trying to use the cdk.Stack class without importing the aws-cdk-lib module.

By following the steps outlined in this article, you should be able to resolve these common issues and work with CDK and CloudFormation effectively and efficiently.

Conclusion

Q: What is the recommended way to import the output values of an existing CloudFormation stack into my CDK stack?

A: The recommended way to import the output values of an existing CloudFormation stack into your CDK stack is by using the core.Fn.import_values method.

Q: How do I use the `core.Fn.import_values` method to import the output values of an existing CloudFormation stack into my CDK stack?

A: To use the core.Fn.import_values method, you need to import the aws-cdk-lib module and then use the import_values method to import the output values of the existing CloudFormation stack. Here's an example of how to do this:

import * as cdk from 'aws-cdk-lib';
import * as iam from 'aws-cdk-lib/aws-iam';
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cloudformation from 'aws-cdk-lib/aws-cloudformation';
export class MyStack extends cdk.Stack {
constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);
// Import the output values of the existing CloudFormation stack
const stackName = &#39;my-existing-stack&#39;;
const outputName = &#39;MyOutput&#39;;
const outputValue = cloudformation.CloudFormationStack.import_values(this, stackName, outputName);

// Use the output value in your CDK stack
const myOutput = outputValue.toString();
console.log(myOutput);

}
}

Q: Can I use the `core.Fn.get_att` method to get the output value of an existing CloudFormation stack?

A: Yes, you can use the core.Fn.get_att method to get the output value of an existing CloudFormation stack. However, this method is not recommended as it can be error-prone and may not work as expected.

Q: How do I use the `core.Fn.get_att` method to get the output value of an existing CloudFormation stack?

A: To use the core.Fn.get_att method, you need to import the aws-cdk-lib module and then use the get_att method to get the output value of the existing CloudFormation stack. Here's an example of how to do this:

import * as cdk from 'aws-cdk-lib';
import * as iam from 'aws-cdk-lib/aws-iam';
import * as ec2 from 'aws-cdk-lib/aws-ec2';
import * as cloudformation from 'aws-cdk-lib/aws-cloudformation';
export class MyStack extends cdk.Stack {
constructor(scope: cdk.Construct, id: string, props?: cdk.StackProps) {
super(scope, id, props);
// Get the output value of the existing CloudFormation stack
const stackName = &#39;my-existing-stack&#39;;
const outputName = &#39;MyOutput&#39;;
const outputValue = cdk.Fn.get_att(`${stackName}.Outputs.${outputName}`);

// Use the output value in your CDK stack
const myOutput = outputValue.toString();
console.log(myOutput);

}
}

Q: What are some common issues that I may encounter when working with CDK and CloudFormation?

A: Some common issues that you may encounter when working with CDK and CloudFormation include:

Error: core.Fn.get_att is not a function: This error occurs when you're trying to use the core.Fn.get_att method without importing the aws-cdk-lib module.
Error: cloudformation.CloudFormationStack is not a function: This error occurs when you're trying to use the cloudformation.CloudFormationStack class without importing the aws-cdk-lib module.
Error: cdk.Stack is not a function: This error occurs when you're trying to use the cdk.Stack class without importing the aws-cdk-lib module.

Q: How do I resolve the common issues that I may encounter when working with CDK and CloudFormation?

A: To resolve the common issues that you may encounter when working with CDK and CloudFormation, you need to make sure that you're importing the aws-cdk-lib module and using the correct methods and classes. Here are some steps that you can follow to resolve the common issues:

Import the aws-cdk-lib module: Make sure that you're importing the aws-cdk-lib module in your CDK stack.
Use the correct methods and classes: Make sure that you're using the correct methods and classes to work with CDK and CloudFormation.
Check the documentation: Check the documentation for CDK and CloudFormation to make sure that you're using the correct methods and classes.
Check the error messages: Check the error messages to see if they provide any clues about what's going wrong.

By following these steps, you should be able to resolve the common issues that you may encounter when working with CDK and CloudFormation.