AWS CDK: how do I reference cross-stack resources in same app?

I have an App that has two stacks, both within the same region/account. One of those stacks requires the ARN of a lambda that exists in the other stack. How do I reference this?

// within stackA constructor
public StackA(Construct scope, String id, StackProps props) {
        SingletonFunction myLambda = SingletonFunction.Builder.create(this, "myLambda")
                                                              // some code here
                                                              .build()
        CfnOutput myLambdaArn = CfnOutput.Builder.create(this, "myLambdaArn")
                                                  .exportName("myLambdaArn")
                                                  .description("ARN of the lambda that I want to use in StackB")
                                                  .value(myLambda.getFunctionArn())
                                                  .build();
    
}

App app = new App();
Stack stackA = new StackA(app, "stackA", someAProps);    
Stack stackB = new StackB(app, "stackB", someBProps);
stackB.dependsOn(stackA);

How do pass the ARN into StackB?

Solution 1:

You can access resources in a different stack, as long as they are in the same account and AWS Region. The following example defines the stack stack1, which defines an Amazon S3 bucket. Then it defines a second stack, stack2, which takes the bucket from stack1 as a constructor property.

// Helper method to build an environment
static Environment makeEnv(String account, String region) {
    return Environment.builder().account(account).region(region)
            .build();
}

App app = new App();

Environment prod = makeEnv("123456789012", "us-east-1");

StackThatProvidesABucket stack1 = new StackThatProvidesABucket(app, "Stack1",
        StackProps.builder().env(prod).build());

// stack2 will take an argument "bucket"
StackThatExpectsABucket stack2 = new StackThatExpectsABucket(app, "Stack,",
        StackProps.builder().env(prod).build(), stack1.getBucket());

Solution 2:

CDK's official documentation has a complete example for sharing a S3 bucket between stacks. I copied it below for quicker reference.

/**
 * Stack that defines the bucket
 */
class Producer extends cdk.Stack {
  public readonly myBucket: s3.Bucket;

  constructor(scope: cdk.App, id: string, props?: cdk.StackProps) {
    super(scope, id, props);

    const bucket = new s3.Bucket(this, 'MyBucket', {
      removalPolicy: cdk.RemovalPolicy.DESTROY,
    });
    this.myBucket = bucket;
  }
}

interface ConsumerProps extends cdk.StackProps {
  userBucket: s3.IBucket;
}

/**
 * Stack that consumes the bucket
 */
class Consumer extends cdk.Stack {
  constructor(scope: cdk.App, id: string, props: ConsumerProps) {
    super(scope, id, props);

    const user = new iam.User(this, 'MyUser');
    props.userBucket.grantReadWrite(user);
  }
}

const producer = new Producer(app, 'ProducerStack');
new Consumer(app, 'ConsumerStack', { userBucket: producer.myBucket });