Implementing automatic drift detection in CDK Pipelines using Amazon EventBridge

The AWS Cloud Development Kit (AWS CDK) is a popular open source toolkit that allows developers to create their cloud infrastructure using high level programming languages. AWS CDK comes bundled with a construct called CDK Pipelines that makes it easy to set up continuous integration, delivery, and deployment with AWS CodePipeline. The CDK Pipelines construct does all the heavy lifting, such as setting up appropriate AWS IAM roles for deployment across regions and accounts, Amazon Simple Storage Service (Amazon S3) buckets to store build artifacts, and an AWS CodeBuild project to build, test, and deploy the app. The pipeline deploys a given CDK application as one or more AWS CloudFormation stacks.

With CloudFormation stacks, there is the possibility that someone can manually change the configuration of stack resources outside the purview of CloudFormation and the pipeline that deploys the stack. This causes the deployed resources to be inconsistent with the intent in the application, which is referred to as “drift”, a situation that can make the application’s behavior unpredictable. For example, when troubleshooting an application, if the application has drifted in production, it is difficult to reproduce the same behavior in a development environment. In other cases, it may introduce security vulnerabilities in the application. For example, an AWS EC2 SecurityGroup that was originally deployed to allow ingress traffic from a specific IP address might potentially be opened up to allow traffic from all IP addresses.

CloudFormation offers a drift detection feature for stacks and stack resources to detect configuration changes that are made outside of CloudFormation. The stack/resource is considered as drifted if its configuration does not match the expected configuration defined in the CloudFormation template and by extension the CDK code that synthesized it.

In this blog post you will see how CloudFormation drift detection can be integrated as a pre-deployment validation step in CDK Pipelines using an event driven approach.

Services and frameworks used in the post include CloudFormation, CodeBuild, Amazon EventBridge, AWS Lambda, Amazon DynamoDB, S3, and AWS CDK.

Solution overview

Amazon EventBridge is a serverless AWS service that offers an agile mechanism for the developers to spin up loosely coupled, event driven applications at scale. EventBridge supports routing of events between services via an event bus. EventBridge out of the box supports a default event bus for each account which receives events from AWS services. Last year, CloudFormation added a new feature that enables event notifications for changes made to CloudFormation-based stacks and resources. These notifications are accessible through Amazon EventBridge, allowing users to monitor and react to changes in their CloudFormation infrastructure using event-driven workflows. Our solution leverages the drift detection events that are now supported by EventBridge. The following architecture diagram depicts the flow of events involved in successfully performing drift detection in CDK Pipelines.

Architecture diagram

The user starts the pipeline by checking code into an AWS CodeCommit repo, which acts as the pipeline source. We have configured drift detection in the pipeline as a custom step backed by a lambda function. When the drift detection step invokes the provider lambda function, it first starts the drift detection on the CloudFormation stack Demo Stack and then saves the drift_detection_id along with pipeline_job_id in a DynamoDB table. In the meantime, the pipeline waits for a response on the status of drift detection.

The EventBridge rules are set up to capture the drift detection state change events for Demo Stack that are received by the default event bus. The callback lambda is registered as the intended target for the rules. When drift detection completes, it triggers the EventBridge rule which in turn invokes the callback lambda function with stack status as either DRIFTED or IN SYNC. The callback lambda function pulls the pipeline_job_id from DynamoDB and sends the appropriate status back to the pipeline, thus propelling the pipeline out of the wait state. If the stack is in the IN SYNC status, the callback lambda sends a success status and the pipeline continues with the deployment. If the stack is in the DRIFTED status, callback lambda sends failure status back to the pipeline and the pipeline run ends up in failure.

Solution Deep Dive

The solution deploys two stacks as shown in the above architecture diagram

1. CDK Pipelines stack
2. Pre-requisite stack

The CDK Pipelines stack defines a pipeline with a CodeCommit source and drift detection step integrated into it. The pre-requisite stack deploys following resources that are required by the CDK Pipelines stack.

• A Lambda function that implements drift detection step
• A DynamoDB table that holds drift_detection_id and pipeline_job_id
• An Event bridge rule to capture “CloudFormation Drift Detection Status Change” event
• A callback lambda function that evaluates status of drift detection and sends status back to the pipeline by looking up the data captured in DynamoDB.

The pre-requisites stack is deployed first, followed by the CDK Pipelines stack.

Defining drift detection step

CDK Pipelines offers a mechanism to define your own step that requires custom implementation. A step corresponds to a custom action in CodePipeline such as invoke lambda function. It can exist as a pre or post deployment action in a given stage of the pipeline. For example, your organization’s policies may require its CI/CD pipelines to run a security vulnerability scan as a prerequisite before deployment. You can build this as a custom step in your CDK Pipelines. In this post, you will use the same mechanism for adding the drift detection step in the pipeline.

You start by defining a class called DriftDetectionStep that extends Step and implements ICodePipelineActionFactory as shown in the following code snippet. The constructor accepts 3 parameters stackName, account, region as inputs. When the pipeline runs the step, it invokes the drift detection lambda function with these parameters wrapped inside userParameters variable. The function produceAction() adds the action to invoke drift detection lambda function to the pipeline stage.

Please note that the solution uses an SSM parameter to inject the lambda function ARN into the pipeline stack. So, we deploy the provider lambda function as part of pre-requisites stack before the pipeline stack and publish its ARN to the SSM parameter. The CDK code to deploy pre-requisites stack can be found here.

export class DriftDetectionStep
    extends Step
    implements pipelines.ICodePipelineActionFactory
{
    constructor(
        private readonly stackName: string,
        private readonly account: string,
        private readonly region: string
    ) {
        super(`DriftDetectionStep-${stackName}`);
    }

    public produceAction(
        stage: codepipeline.IStage,
        options: ProduceActionOptions
    ): CodePipelineActionFactoryResult {
        // Define the configuraton for the action that is added to the pipeline.
        stage.addAction(
            new cpactions.LambdaInvokeAction({
                actionName: options.actionName,
                runOrder: options.runOrder,
                lambda: lambda.Function.fromFunctionArn(
                    options.scope,
                    `InitiateDriftDetectLambda-${this.stackName}`,
                    ssm.StringParameter.valueForStringParameter(
                        options.scope,
                        SSM_PARAM_DRIFT_DETECT_LAMBDA_ARN
                    )
                ),
                // These are the parameters passed to the drift detection step implementaton provider lambda
                userParameters: {
                    stackName: this.stackName,
                    account: this.account,
                    region: this.region,
                },
            })
        );
        return {
            runOrdersConsumed: 1,
        };
    }
}

Configuring drift detection step in CDK Pipelines

Here you will see how to integrate the previously defined drift detection step into CDK Pipelines. The pipeline has a stage called DemoStage as shown in the following code snippet. During the construction of DemoStage, we declare drift detection as the pre-deployment step. This makes sure that the pipeline always does the drift detection check prior to deployment.

Please note that for every stack defined in the stage; we add a dedicated step to perform drift detection by instantiating the class DriftDetectionStep detailed in the prior section. Thus, this solution scales with the number of stacks defined per stage.

export class PipelineStack extends BaseStack {
    constructor(scope: Construct, id: string, props?: StackProps) {
        super(scope, id, props);

        const repo = new codecommit.Repository(this, 'DemoRepo', {
            repositoryName: `${this.node.tryGetContext('appName')}-repo`,
        });

        const pipeline = new CodePipeline(this, 'DemoPipeline', {
            synth: new ShellStep('synth', {
                input: CodePipelineSource.codeCommit(repo, 'main'),
                commands: ['./script-synth.sh'],
            }),
            crossAccountKeys: true,
            enableKeyRotation: true,
        });
        const demoStage = new DemoStage(this, 'DemoStage', {
            env: {
                account: this.account,
                region: this.region,
            },
        });
        const driftDetectionSteps: Step[] = [];
        for (const stackName of demoStage.stackNameList) {
            const step = new DriftDetectionStep(stackName, this.account, this.region);
            driftDetectionSteps.push(step);
        }
        pipeline.addStage(demoStage, {
            pre: driftDetectionSteps,
        });

Demo

Here you will go through the deployment steps for the solution and see drift detection in action.

Deploy the pre-requisites stack

Clone the repo from the GitHub location here. Navigate to the cloned folder and run script script-deploy.sh You can find detailed instructions in README.md

Deploy the CDK Pipelines stack

Clone the repo from the GitHub location here. Navigate to the cloned folder and run script script-deploy.sh. This deploys a pipeline with an empty CodeCommit repo as the source. The pipeline run ends up in failure, as shown below, because of the empty CodeCommit repo.

Next, check in the code from the cloned repo into the CodeCommit source repo. You can find detailed instructions on that in README.md  This triggers the pipeline and pipeline finishes successfully, as shown below.

The pipeline deploys two stacks DemoStackA and DemoStackB. Each of these stacks creates an S3 bucket.

Demonstrate drift detection

Locate the S3 bucket created by DemoStackA under resources, navigate to the S3 bucket and modify the tag aws-cdk:auto-delete-objects from true to false as shown below

Now, go to the pipeline and trigger a new execution by clicking on Release Change

The pipeline run will now end in failure at the pre-deployment drift detection step.

Cleanup

Please follow the steps below to clean up all the stacks.

1. Navigate to S3 console and empty the buckets created by stacks DemoStackA and DemoStackB.
2. Navigate to the CloudFormation console and delete stacks DemoStackA and DemoStackB, since deleting CDK Pipelines stack does not delete the application stacks that the pipeline deploys.
3. Delete the CDK Pipelines stack cdk-drift-detect-demo-pipeline
4. Delete the pre-requisites stack cdk-drift-detect-demo-drift-detection-prereq

Conclusion

In this post, I showed how to add a custom implementation step in CDK Pipelines. I also used that mechanism to integrate a drift detection check as a pre-deployment step. This allows us to validate the integrity of a CloudFormation Stack before its deployment. Since the validation is integrated into the pipeline, it is easier to manage the solution in one place as part of the overarching pipeline. Give the solution a try, and then see if you can incorporate it into your organization’s delivery pipelines.

About the author:

Damodar Shenvi Wagle is a Senior Cloud Application Architect at AWS Professional Services. His areas of expertise include architecting serverless solutions, CI/CD, and automation.