CDK Stack Notification Options

Today, I discovered yet again that there are countless ways to tackle a single task as a developer.

I was tasked with automating a workflow that involved an AWS Lambda Function triggered by an SNS event source. The goal was to publish a message to an SNS topic in a different AWS account when the status of a CloudFormation stack updated.
We use AWS CDK for infrastructure as code (IAC). While exploring the documentation and blog posts, I found that there is no direct equivalent of the Notification Policy in CloudFormation to publish notifications to an SNS topic on a CloudFormation stack status change. Instead, there are several common patterns to achieve this. Let's start with solution diagram. Here's a simplified version of the architecture diagram of what I implemented:
If this is what you are looking for, you can simply achieve it in 3 different ways:

Using AWS Event Bridge

• Create SNS Topic as a Stack B CDK resource:

import { Topic } from "aws-cdk-lib/aws-sns";

   const SNSTopic = new Topic(this, "SNS_TOPIC_ID", {
      displayName: "YOUR DISPLAY NAME",
    }); 

_Note: you need to add an event source to your function or any other resources that is going to subscribe to this topic. in my case I needed to configure a lambda event source as _bellow:

MyFunction.addEventSource(new SnsEventSource(SNSTopic));

• Add Event Rule

 new Rule(this, "Trigger", {
      eventPattern: {
        source: ["aws.cloudformation"],
        detailType: ["CloudFormation Stack Status Change"],
        detail: {
          eventName: ["CREATE_COMPLETE", "UPDATE_COMPLETE", "DELETE_COMPLETE"],
          requestParameters: {
            stackName: [this.stackName],
          },
        },
      },
      targets: [new SnsTopic(SNSTopic)],
    });

Using AWS Custom Resources

Second approach to achieve this is by using AWS Custom Resources.

• Create an AWS Custom Resource within the Stack B CDK:

import { AwsCustomResource, AwsCustomResourcePolicy, PhysicalResourceId } from "aws-cdk-lib/custom-resources";    

const Trigger = new AwsCustomResource(this, "TriggerOnSuccess", {
      onUpdate: {
        service: "SNS",
        action: "publish",
        parameters: {
          TopicArn: "YOUR_TOPIC_ARN",
          Message: "Stack updated successfully",
        },
        physicalResourceId: PhysicalResourceId.of("TriggerOnSuccess"),
      },
      onDelete: {
        service: "SNS",
        action: "publish",
        parameters: {
          TopicArn: "YOUR_TOPIC_ARN",
          Message: "Stack deleted successfully",
        },
        physicalResourceId: PhysicalResourceId.of("TriggerOnSuccess"),
      },
       policy: AwsCustomResourcePolicy.fromStatements( [new PolicyStatement({
        actions: ["sns:Publish"],
        effect: Effect.ALLOW,
        resources: [SNSTopic.topicArn],
      })]),
    });

• Add Dependency order so that CDK doesn't return Dependency Cycle error

 Trigger.node.addDependency(YOUR_FUNCTION);
 Trigger.node.addDependency(SNSS_TOPIC); 

Using AWS Custom Resource with Lambda Invoke Action

There is also a 3rd solution for this as well which I am not a big fan of it and that is. I personally prefer to use fan out approach, to populate events to Lambda via an "Event Service" such as SNS or EventBridge. If you look for a simplified Custom Resource, here is what you should update your AWS Custom Resource to:

import { AwsCustomResource, AwsCustomResourcePolicy, PhysicalResourceId } from "aws-cdk-lib/custom-resources";    

const Trigger = new AwsCustomResource(this, "TriggerOnSuccess", {
      onUpdate: {
        service: "Lambda",
        action: "invoke",
        parameters: {
          FunctionName: "YOUR_FUNCTION_ARN",
          InvokationType: "Event"
        },
        physicalResourceId: PhysicalResourceId.of("TriggerOnSuccess"),
      },
      onDelete: {
        service: "Lambda",
        action: "invoke",
        parameters: {
          FunctionName: "YOUR_FUNCTION_ARN",
          InvokationType: "Event"
        },
        physicalResourceId: PhysicalResourceId.of("TriggerOnSuccess"),
      },
       policy: AwsCustomResourcePolicy.fromStatements( [new PolicyStatement({
        actions: ["lambda:InvokeFunction"],
        effect: Effect.ALLOW,
        resources: [YOUR_FUNCTION_ARN],
      })]),
    });

This concludes our brief discussion. While I'm still hopeful about discovering if CDK offers an API for configuring Stack Notification Options, I wanted to share these workarounds in the meantime.

Comments

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