🔧 When automation backfires: How CDK defaults can create customer-facing issues

The Problem

AWS CDK makes infrastructure deployment incredibly easy, but with great power comes great responsibility. One area where this is particularly evident is CloudFront TTL (Time To Live) configuration. A seemingly innocent default setting can lead to frustrated users and damaged customer trust.

What Happened

When deploying a static website using CDK’s CloudFrontWebDistribution construct, many developers rely on default cache behaviors. However, these defaults might not align with your application’s needs:

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// ❌ Problematic: Using defaults without consideration
const distribution = new cloudfront.CloudFrontWebDistribution(this, 'Distribution', {
  originConfigs: [{
    s3OriginSource: {
      s3BucketSource: bucket
    },
    behaviors: [{ isDefaultBehavior: true }]
  }]
});

The default TTL settings can cause:

  • Stale content delivery to users
  • Delayed updates during critical deployments
  • Inconsistent user experiences across regions
  • Customer support overhead from confused users

The Solution: Intentional Cache Configuration

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// ✅ Better: Explicit cache configuration
const distribution = new cloudfront.CloudFrontWebDistribution(this, 'Distribution', {
  originConfigs: [{
    s3OriginSource: {
      s3BucketSource: bucket
    },
    behaviors: [{
      isDefaultBehavior: true,
      minTtl: Duration.seconds(0),
      defaultTtl: Duration.minutes(5),
      maxTtl: Duration.hours(1),
      cachedMethods: cloudfront.CloudFrontAllowedCachedMethods.GET_HEAD_OPTIONS,
      compress: true
    }]
  }]
});

Best Practices for CDK CloudFront Configuration

1. Differentiate Content Types

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// Static assets: Long cache
{
  pathPattern: "/assets/*",
  minTtl: Duration.days(1),
  defaultTtl: Duration.days(30),
  maxTtl: Duration.days(365)
}

// HTML pages: Short cache
{
  pathPattern: "*.html",
  minTtl: Duration.seconds(0),
  defaultTtl: Duration.minutes(5),
  maxTtl: Duration.hours(1)
}

// API responses: No cache
{
  pathPattern: "/api/*",
  minTtl: Duration.seconds(0),
  defaultTtl: Duration.seconds(0),
  maxTtl: Duration.seconds(0)
}

2. Implement Cache Invalidation Strategy

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// Add invalidation to your deployment pipeline
const invalidation = new cloudfront.CfnInvalidation(this, 'Invalidation', {
  distributionId: distribution.distributionId,
  invalidationBatch: {
    paths: ['/*'],
    callerReference: Date.now().toString()
  }
});

3. Monitor Cache Performance

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// Add CloudWatch alarms for cache hit ratio
new cloudwatch.Alarm(this, 'CacheHitRatioAlarm', {
  metric: distribution.metricCacheHitRate(),
  threshold: 85,
  comparisonOperator: cloudwatch.ComparisonOperator.LESS_THAN_THRESHOLD
});

Key Takeaways

  1. Never rely on defaults for customer-facing infrastructure
  2. Test cache behavior in staging environments
  3. Monitor cache metrics to ensure optimal performance
  4. Document your cache strategy for team members
  5. Plan invalidation strategies for emergency deployments

Conclusion

CDK’s power lies in its ability to codify infrastructure, but this requires intentional decision-making. By explicitly configuring CloudFront TTL settings and implementing proper cache strategies, you can maintain customer trust while leveraging the benefits of CDN caching.

Remember: automation should enhance reliability, not introduce new failure modes.

Ivan Yalovets

Ivan Yalovets

AWS Cloud Architect & DevOps Engineer

Have you encountered similar issues with CDK defaults? Share your experiences in the comments or reach out on LinkedIn.