How to Get Attribute [GetAtt] in AWS CDK

GetAtt in CDK #

The GetAtt function returns the value of a specific attribute.

GetAtt is a function that comes from CloudFormation, but since our CDK code gets compiled down to CloudFormation before a deployment, we can use this feature in CDK as well.

We can use the GetAtt function in CDK in two ways:

• Use it directly on the CfnResource, by calling the getAtt function and passing in the attribute name
• Use it by invoking the getAtt function on the core Fn class

In this article we are going to demonstrate how to use GetAtt both ways. I'll create a simple CDK stack consisting of a single S3 bucket.

GetAtt directly on the CfnResource #

First, we'll use the getAtt function directly on the CfnResource:

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import * as s3 from 'aws-cdk-lib/aws-s3';
import * as cdk from 'aws-cdk-lib';

export class MyCdkStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props: cdk.StackProps) {
    super(scope, id, props);

    const s3Bucket = new s3.Bucket(this, id);
    const cfnBucket = s3Bucket.node.defaultChild as s3.CfnBucket;

    console.log('directly 👉', cfnBucket.getAtt('Arn').toString());
  }
}

In the code snippet, we:

1. Defined an S3 bucket using the level 2 Bucket construct

2. Used the node.defaultChild property and casted the type to a CfnBucket so we can access the getAtt method

3. Logged the call to getAtt as a string

If we synth the stack at this point, we'll see that the value resolves to a Token.

In short a token is an encoded value that will be resolved by CloudFormation at deployment time. If you want to read more on tokens I have written an article - What is a Token in CDK.

GetAtt using the Fn class #

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import * as s3 from 'aws-cdk-lib/aws-s3';
import * as cdk from 'aws-cdk-lib';

export class MyCdkStack extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props: cdk.StackProps) {
    super(scope, id, props);

    // ...rest

    console.log(
      'on Fn class 👉',
      cdk.Fn.getAtt(cfnBucket.logicalId, 'Arn').toString(),
    );

    // 👇 add the call to getAtt as an Output
    new cdk.CfnOutput(this, 'bucketArn', {
      value: cfnBucket.getAtt('Arn').toString(),
      description: 'The arn of the s3 bucket',
      exportName: 'avatarsBucket',
    });
  }
}

In the code snippet, we:

1. Used the getAtt static method on the Fn class. The method takes the CloudFormation logical ID of the resource as the first parameter and the name of the attribute as the second.

2. We created an Output and specified the bucket's arn from the call to getAtt as the value.

The output from the console.log statement is a Token that will get resolved by CloudFormation at deployment time:

Let's deploy the stack and see if the call to cfnBucket.getAtt resolves in the Outputs section:

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npx aws-cdk deploy \
  --outputs-file ./cdk-outputs.json

After I've ran the deploy command with the outputs redirected to a file on the local file system, the contents of cdk-outputs.json look like:

We can see that the call to cfnBucket.getAtt successfully got resolved by CloudFormation at deployment time.

The cdk.out directory is where the CDK CLI stores file assets in preparation for deployment. If we take a look at the synthesized CloudFormation template, we can see the use of the intrinsic GetAtt function:

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{
  "Outputs": {
    "bucketArn": {
      "Description": "The arn of the s3 bucket",
      "Value": {
        // 👇 Uses GetAtt intrinsic function
        "Fn::GetAtt": [
          "mycdkstack4E08F0DD",
          "Arn"
        ]
      },
      "Export": {
        "Name": "avatarsBucket"
      }
    }
  }
}

Further Reading #

• AWS CDK Tutorial for Beginners - Step-by-Step Guide
• How does AWS CDK work
• What is a Token in AWS CDK
• CDK Constructs - Complete Guide
• What is an identifier (id) in AWS CDK
• How to use Context in AWS CDK
• How to use Parameters in AWS CDK