SNAPSHOT_TESTING.md

CDK Snapshot Testing Guide

This guide explains how to use the asset hash normalization utility for consistent CDK snapshot testing across different environments.

Problem

CDK snapshot tests often fail in CI/CD environments because asset hashes change between different environments. This happens because:

• Asset hashes are generated based on content and environment variables
• Different machines or CI/CD environments produce different hashes
• This causes snapshot tests to fail even when there are no actual changes to the infrastructure

Solution

We've created a utility function that normalizes asset hashes and other environment-specific values in CloudFormation templates before comparing them with snapshots.

How to Use the Normalization Utility

1. Import the Utility

import { normalizeTemplate } from '../../test-utils/normalize-template';

2. Apply Normalization Before Snapshot Comparison

import { App } from 'aws-cdk-lib';
import { Template } from 'aws-cdk-lib/assertions';
import { YourStack } from '../lib/your-stack';
import { normalizeTemplate } from '../../test-utils/normalize-template';

test('YourStack creates the expected resources', () => {
  const app = new App();
  const stack = new YourStack(app, 'TestStack');
  
  // Get the CloudFormation template
  const template = Template.fromStack(stack);
  
  // Normalize the template before snapshot comparison
  const normalizedTemplate = normalizeTemplate(template.toJSON());
  
  // Compare with snapshot
  expect(normalizedTemplate).toMatchSnapshot();
});

3. Update Existing Snapshots

After adding the normalization to your tests, update your snapshots:

npm test -- -u

What Gets Normalized

The utility currently normalizes:

1. S3 Asset Keys: Replaces asset hashes in S3Key properties

   • Pattern with extension: [64 hex chars].zip → NORMALIZED_ASSET_HASH.zip
   • Pattern without extension: [64 hex chars] → NORMALIZED_ASSET_HASH

2. Docker Image Digests: Replaces image digests

   • Pattern: sha256:[digest] → NORMALIZED_IMAGE_DIGEST

Adding New Test Files

When creating new test files that use snapshot testing:

1. Import the normalization utility
2. Apply it to your template before comparing with snapshots
3. Update your snapshots with the -u flag

Extending the Utility

If you encounter other environment-specific values that need normalization, you can extend the utility at typescript/test-utils/normalize-template.ts.

Example of adding a new normalization rule:

// Inside the normalizeValues function
if (key === 'NewPropertyToNormalize' && typeof obj[key] === 'string' && /pattern-to-match/.test(obj[key])) {
  obj[key] = 'NORMALIZED_VALUE';
}

Troubleshooting

If you're still seeing snapshot test failures:

1. Check for new patterns: There might be new types of asset hashes or environment-specific values that need normalization
2. Verify imports: Make sure you're importing and using the utility correctly
3. Check snapshot updates: Ensure you've updated your snapshots after adding normalization

Best Practices

1. Always normalize before snapshot comparison: This ensures consistent results
2. Update snapshots deliberately: Only use the -u flag when you expect changes
3. Review snapshot diffs: When updating snapshots, review the changes to ensure they're expected
4. Keep the utility updated: As new patterns emerge, add them to the normalization utility

Additional Resources

• Jest Snapshot Testing Documentation
• AWS CDK Testing Documentation