aws-cdk.aws-cloudformation 1.5.0

CDK Constructs for AWS CloudFormation

AWS CloudFormation Construct Library

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This module is part of the AWS Cloud Development Kit project.

Custom Resources

Custom Resources are CloudFormation resources that are implemented by arbitrary user code. They can do arbitrary lookups or modifications during a CloudFormation synthesis run.

You will typically use Lambda to implement a Construct implemented as a Custom Resource (though SNS topics can be used as well). Your Lambda function will be sent a CREATE, UPDATE or DELETE message, depending on the CloudFormation life cycle. It will perform whatever actions it needs to, and then return any number of output values which will be available as attributes of your Construct. In turn, those can be used as input to other Constructs in your model.

In general, consumers of your Construct will not need to care whether it is implemented in term of other CloudFormation resources or as a custom resource.

Note: when implementing your Custom Resource using a Lambda, use a SingletonLambda so that even if your custom resource is instantiated multiple times, the Lambda will only get uploaded once.

Example

The following shows an example of a declaring Custom Resource that copies files into an S3 bucket during deployment (the implementation of the actual Lambda handler is elided for brevity).

interface CopyOperationProps {
  sourceBucket: IBucket;
  targetBucket: IBucket;
}

class CopyOperation extends Construct {
  constructor(parent: Construct, name: string, props: CopyOperationProps) {
    super(parent, name);

    const lambdaProvider = new lambda.SingletonFunction(this, 'Provider', {
      uuid: 'f7d4f730-4ee1-11e8-9c2d-fa7ae01bbebc',
      runtime: lambda.Runtime.PYTHON_3_7,
      code: lambda.Code.fromAsset('../copy-handler'),
      handler: 'index.handler',
      timeout: Duration.seconds(60),
    });

    new CustomResource(this, 'Resource', {
      provider: CustomResourceProvider.lambda(lambdaProvider),
      properties: {
        sourceBucketArn: props.sourceBucket.bucketArn,
        targetBucketArn: props.targetBucket.bucketArn,
      }
    });
  }
}

The aws-cdk-examples repository has examples for adding custom resources.

References

See the following section of the docs on details to write Custom Resources:

• Introduction
• Reference
• Code Reference