cdk-diff-pr-github-action

A small Projen-based helper library that wires GitHub workflows for:

• Creating CloudFormation Change Sets for your CDK stacks on pull requests and commenting a formatted diff back on the PR.
• Detecting CloudFormation drift on a schedule or manual trigger and producing a consolidated summary (optionally creating an issue).

It also provides ready‑to‑deploy IAM templates with the minimal permissions required for each workflow.

This package exposes four constructs:

• CdkDiffStackWorkflow — Generates one GitHub Actions workflow per stack to create a change set and render the diff back to the PR and Step Summary.
• CdkDiffIamTemplate — Emits a CloudFormation template file with minimal permissions for the Change Set workflow.
• CdkDriftDetectionWorkflow — Generates a GitHub Actions workflow to detect CloudFormation drift per stack, upload machine‑readable results, and aggregate a summary.
• CdkDriftIamTemplate — Emits a CloudFormation template file with minimal permissions for the Drift Detection workflow.

Quick start

1. Add the constructs to your Projen project (in .projenrc.ts).
2. Synthesize with npx projen.
3. Commit the generated files.
4. Open a pull request or run the drift detection workflow.

Usage: CdkDiffStackWorkflow

CdkDiffStackWorkflow renders a workflow per stack named diff-<StackName>.yml under .github/workflows/. It also generates a helper script at .github/workflows/scripts/describe-cfn-changeset.ts that formats the change set output and takes care of posting the PR comment and Step Summary.

Example .projenrc.ts:

import { awscdk } from 'projen';
import { CdkDiffStackWorkflow } from '@jjrawlins/cdk-diff-pr-github-action';

const project = new awscdk.AwsCdkConstructLibrary({
  // ... your usual settings ...
  workflowName: 'my-lib',
  defaultReleaseBranch: 'main',
  cdkVersion: '2.85.0',
  github: true,
});

new CdkDiffStackWorkflow({
  project,
  stacks: [
    {
      stackName: 'MyAppStack',
      changesetRoleToAssumeArn: 'arn:aws:iam::123456789012:role/cdk-diff-role',
      changesetRoleToAssumeRegion: 'us-east-1',
      // Optional per‑stack OIDC override (if not using the defaults below)
      // oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
      // oidcRegion: 'us-east-1',
    },
  ],
  // Default OIDC role/region used by all stacks unless overridden per‑stack
  oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
  oidcRegion: 'us-east-1',
  // Optional: Node version used in the workflow (default: '24.x')
  // nodeVersion: '24.x',
  // Optional: Yarn command to run CDK (default: 'cdk')
  // cdkYarnCommand: 'cdk',
  // Optional: Where to place the helper script (default: '.github/workflows/scripts/describe-cfn-changeset.ts')
  // scriptOutputPath: '.github/workflows/scripts/describe-cfn-changeset.ts',
});

project.synth();

CdkDiffStackWorkflow props

• project (required) — Your Projen project instance.
• stacks (required) — Array of stack entries.
• oidcRoleArn (required unless provided per‑stack) — Default OIDC role ARN.
• oidcRegion (required unless provided per‑stack) — Default OIDC region.
• nodeVersion (optional, default '24.x') — Node.js version for the workflow runner.
• cdkYarnCommand (optional, default 'cdk') — Yarn script/command to invoke CDK.
• scriptOutputPath (optional, default '.github/workflows/scripts/describe-cfn-changeset.ts') — Where to write the helper script.

If neither top‑level OIDC defaults nor all per‑stack values are supplied, the construct throws a helpful error.

Stack item fields

• stackName (required) — The CDK stack name to create the change set for.
• changesetRoleToAssumeArn (required) — The ARN of the role used to create the change set (role chaining after OIDC).
• changesetRoleToAssumeRegion (required) — The region for that role.
• oidcRoleArn (optional) — Per‑stack override for the OIDC role.
• oidcRegion (optional) — Per‑stack override for the OIDC region.

What gets generated

• .github/workflows/diff-<StackName>.yml — One workflow per stack, triggered on PR open/sync/reopen.
• .github/workflows/scripts/describe-cfn-changeset.ts — A helper script that:
  • Polls DescribeChangeSet until terminal
  • Filters out ignorable logical IDs or resource types using environment variables IGNORE_LOGICAL_IDS and IGNORE_RESOURCE_TYPES
  • Renders an HTML table with actions, logical IDs, types, replacements, and changed properties
  • Prints the HTML, appends to the GitHub Step Summary, and (if GITHUB_TOKEN and GITHUB_COMMENT_URL are present) posts a PR comment

Environment variables used by the change set script

• STACK_NAME (required) — Stack name to describe.
• CHANGE_SET_NAME (default: same as STACK_NAME).
• AWS_REGION — Region for CloudFormation API calls. The workflow sets this via the credentials action(s).
• GITHUB_TOKEN (optional) — If set with GITHUB_COMMENT_URL, posts a PR comment.
• GITHUB_COMMENT_URL (optional) — PR comments URL.
• GITHUB_STEP_SUMMARY (optional) — When present, appends the HTML to the step summary file.
• IGNORE_LOGICAL_IDS (optional) — Comma‑separated logical IDs to ignore (default includes CDKMetadata).
• IGNORE_RESOURCE_TYPES (optional) — Comma‑separated resource types to ignore (e.g., AWS::CDK::Metadata).

Usage: CdkDiffIamTemplate

Emit an example IAM template you can deploy in your account for the Change Set workflow:

import { awscdk } from 'projen';
import { CdkDiffIamTemplate } from '@jjrawlins/cdk-diff-pr-github-action';

const project = new awscdk.AwsCdkConstructLibrary({
  // ...
});

new CdkDiffIamTemplate({
  project,
  roleName: 'cdk-diff-role',
  oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
  oidcRegion: 'us-east-1',
  // Optional: custom output path (default: 'cdk-diff-workflow-iam-template.yaml')
  // outputPath: 'infra/cdk-diff-iam.yaml',
});

project.synth();

This writes cdk-diff-workflow-iam-template.yaml at the project root (or your chosen outputPath). The template defines:

• Parameter GitHubOIDCRoleArn with a default from oidcRoleArn — the ARN of your existing GitHub OIDC role allowed to assume the change set role.
• IAM role CdkChangesetRole with minimal permissions for:
  • CloudFormation Change Set operations
  • Access to common CDK bootstrap S3 buckets and SSM parameters
  • iam:PassRole to cloudformation.amazonaws.com
• Outputs exporting the role name and ARN.

A Projen task is also added:

npx projen deploy-cdkdiff-iam-template -- --parameter-overrides GitHubOIDCRoleArn=... # plus any extra AWS CLI args

Use the created role ARN as changesetRoleToAssumeArn in CdkDiffStackWorkflow.

---

Usage: CdkDriftDetectionWorkflow

CdkDriftDetectionWorkflow creates a single workflow file (default drift-detection.yml) that can run on a schedule and via manual dispatch. It generates a helper script at .github/workflows/scripts/detect-drift.ts (by default) that uses AWS SDK v3 to run drift detection, write optional machine‑readable JSON, and print an HTML report for the Step Summary.

Example .projenrc.ts:

import { awscdk } from 'projen';
import { CdkDriftDetectionWorkflow } from '@jjrawlins/cdk-diff-pr-github-action';

const project = new awscdk.AwsCdkConstructLibrary({ github: true, /* ... */ });

new CdkDriftDetectionWorkflow({
  project,
  workflowName: 'Drift Detection',            // optional; file name derived as 'drift-detection.yml'
  schedule: '0 1 * * *',                      // optional cron
  createIssues: true,                         // default true; create/update issue when drift detected on schedule
  oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
  oidcRegion: 'us-east-1',
  // Optional: Node version (default '24.x')
  // nodeVersion: '24.x',
  // Optional: Where to place the helper script (default '.github/workflows/scripts/detect-drift.ts')
  // scriptOutputPath: '.github/workflows/scripts/detect-drift.ts',
  stacks: [
    {
      stackName: 'MyAppStack-Prod',
      driftDetectionRoleToAssumeArn: 'arn:aws:iam::123456789012:role/cdk-drift-role',
      driftDetectionRoleToAssumeRegion: 'us-east-1',
      // failOnDrift: true, // optional (default true)
    },
  ],
});

project.synth();

CdkDriftDetectionWorkflow props

• project (required) — Your Projen project instance.
• stacks (required) — Array of stacks to check.
• oidcRoleArn (required) — Default OIDC role ARN used before chaining into per‑stack drift roles.
• oidcRegion (required) — Default OIDC region.
• workflowName (optional, default 'drift-detection') — Human‑friendly workflow name; the file name is derived in kebab‑case.
• schedule (optional) — Cron expression for automatic runs.
• createIssues (optional, default true) — When true, scheduled runs will create/update a GitHub issue if drift is detected.
• nodeVersion (optional, default '24.x') — Node.js version for the runner.
• scriptOutputPath (optional, default '.github/workflows/scripts/detect-drift.ts') — Where to write the helper script.

Per‑stack fields

• stackName (required) — The full CloudFormation stack name.
• driftDetectionRoleToAssumeArn (required) — Role to assume (after OIDC) for making drift API calls.
• driftDetectionRoleToAssumeRegion (required) — Region for that role and API calls.
• failOnDrift (optional, default true) — Intended to fail the detection step on drift. The provided script exits with non‑zero when drift is found; the job continues to allow artifact upload and issue creation.

What gets generated

• .github/workflows/<kebab(workflowName)>.yml — A workflow with one job per stack plus a final summary job.
• .github/workflows/scripts/detect-drift.ts — Helper script that:
  • Starts drift detection and polls until completion
  • Lists non‑IN_SYNC resources and builds an HTML report
  • Writes optional JSON to DRIFT_DETECTION_OUTPUT when set
  • Prints to stdout and appends to the GitHub Step Summary when available

Artifacts and summary

• Each stack job uploads drift-results-<stack>.json (if produced).
• A final Drift Detection Summary job downloads all artifacts and prints a consolidated summary.

Manual dispatch

• The workflow exposes an input named stack with choices including each configured stack and an all option.
• Choose a specific stack to run drift detection for that stack only, or select all (or leave the input empty) to run all stacks.

Note: The default workflow does not post PR comments for drift. It can create/update an Issue on scheduled runs when createIssues is true.

Post-notification steps (e.g., Slack)

You can add your own GitHub Action steps to run after the drift detection step for each stack using postGitHubSteps. Provide your own Slack payload/markdown (this library no longer generates a payload step for you).

Option A: slackapi/slack-github-action (Incoming Webhook, official syntax)

new CdkDriftDetectionWorkflow({
  project,
  oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
  oidcRegion: 'us-east-1',
  stacks: [/* ... */],
  postGitHubSteps: ({ stack }) => {
    // Build a descriptive name per stack
    const name = `Notify Slack (${stack} post-drift)`;
    const step = {
      name,
      uses: 'slackapi/slack-github-action@v2.1.1',
      // by default, post steps run only when drift is detected; you can override `if`
      if: "always() && steps.drift.outcome == 'failure'",
      // Use official inputs: webhook + webhook-type, and a YAML payload with blocks
      with: {
        webhook: '${{ secrets.CDK_NOTIFICATIONS_SLACK_WEBHOOK }}',
        'webhook-type': 'incoming-webhook',
        payload: [
          'text: "** ${{ env.STACK_NAME }} ** has drifted!"',
          'blocks:',
          '  - type: "section"',
          '    text:',
          '      type: "mrkdwn"',
          '      text: "*Stack:* ${{ env.STACK_NAME }} (region ${{ env.AWS_REGION }}) has drifted:exclamation:"',
          '  - type: "section"',
          '    fields:',
          '      - type: "mrkdwn"',
          '        text: "*Stack ARN*\\n${{ steps.drift.outputs.stack-arn }}"',
          '      - type: "mrkdwn"',
          '        text: "*Issue*\\n<${{ github.server_url }}/${{ github.repository }}/issues/${{ steps.issue.outputs.result }}|#${{ steps.issue.outputs.result }}>"',
        ].join('\n'),
      },
    };
    return [step];
  },
});

Note: The Issue link requires createIssues: true (default) so that the Create Issue on Drift step runs before this Slack step and exposes steps.issue.outputs.result. This library orders the steps accordingly.

Details:

• postGitHubSteps can be:
  • an array of step objects, or
  • a factory function ({ stack }) => step | step[].
• Each step you provide is inserted after the results are uploaded.
• Default condition: if you do not set if on your step, it will default to always() && steps.drift.outcome == 'failure'.
• Available context/env you can use:
  • ${{ env.STACK_NAME }}, ${{ env.DRIFT_DETECTION_OUTPUT }}
  • ${{ steps.drift.outcome }} — success/failure of the detect step
  • ${{ steps.drift.outputs.stack-arn }} — Stack ARN resolved at runtime
  • ${{ steps.issue.outputs.result }} — Issue number if the workflow created/found one (empty when not applicable)

## Usage: CdkDriftIamTemplate

Emit an example IAM template you can deploy in your account for the Drift Detection workflow:

```ts
import { awscdk } from 'projen';
import { CdkDriftIamTemplate } from '@jjrawlins/cdk-diff-pr-github-action';

const project = new awscdk.AwsCdkConstructLibrary({
  // ...
});

new CdkDriftIamTemplate({
  project,
  roleName: 'cdk-drift-role',
  oidcRoleArn: 'arn:aws:iam::123456789012:role/github-oidc-role',
  oidcRegion: 'us-east-1',
  // Optional: custom output path (default: 'cdk-drift-workflow-iam-template.yaml')
  // outputPath: 'infra/cdk-drift-iam.yaml',
});

project.synth();

This writes cdk-drift-workflow-iam-template.yaml at the project root (or your chosen outputPath). The template defines:

• Parameter GitHubOIDCRoleArn with a default from oidcRoleArn — the ARN of your existing GitHub OIDC role allowed to assume this drift role.
• IAM role CdkDriftRole with minimal permissions for CloudFormation drift detection operations.
• Outputs exporting the role name and ARN.

A Projen task is also added:

npx projen deploy-cdkdrift-iam-template -- --parameter-overrides GitHubOIDCRoleArn=... # plus any extra AWS CLI args

Testing

This repository includes Jest tests that snapshot the synthesized outputs from Projen and assert that:

• Diff workflows are created per stack and contain all expected steps.
• Drift detection workflow produces one job per stack and a summary job.
• Only one helper script file is generated per workflow type.
• Per‑stack OIDC overrides (where supported) are respected.
• Helpful validation errors are thrown for missing OIDC settings.
• The IAM template files contain the expected resources and outputs.

Run tests with:

yarn test

Notes

• This package assumes your repository is configured with GitHub Actions and that you have a GitHub OIDC role configured in AWS.
• The generated scripts use the AWS SDK v3 for CloudFormation and, where applicable, the GitHub REST API.