AWS CloudFront: Content Delivery Network Optimization

Amazon CloudFront is AWS's globally distributed content delivery network (CDN) that caches content at edge locations close to your users. While spinning up a basic distribution is straightforward, extracting maximum performance and cost efficiency requires deliberate optimization. This post covers practical techniques to get the most out of CloudFront.

Why CloudFront Optimization Matters

An unoptimized CDN can result in low cache hit ratios, high origin load, inflated data transfer costs, and slower response times for end users. The goal of optimization is simple: serve more requests from the edge, reduce origin round-trips, and control costs.

1. Maximize Your Cache Hit Ratio

The single most impactful metric is your cache hit ratio—the percentage of requests served from the edge instead of the origin.

Normalize Cache Keys

By default, every unique combination of query strings, headers, and cookies can create a separate cache entry. Fragmenting your cache reduces hit ratios. Use Cache Policies to include only the values that actually affect the response.

{
  "CachePolicyConfig": {
    "Name": "OptimizedPolicy",
    "DefaultTTL": 86400,
    "MaxTTL": 31536000,
    "MinTTL": 1,
    "ParametersInCacheKeyAndForwardedToOrigin": {
      "QueryStringsConfig": {
        "QueryStringBehavior": "whitelist",
        "QueryStrings": { "Items": ["version"], "Quantity": 1 }
      },
      "HeadersConfig": { "HeaderBehavior": "none" },
      "CookiesConfig": { "CookieBehavior": "none" }
    }
  }
}

Set Appropriate TTLs

Control caching duration through origin Cache-Control headers, or override them with CloudFront TTL settings:

Cache-Control: public, max-age=86400, s-maxage=604800

Use s-maxage to instruct CloudFront (a shared cache) to cache longer than browsers.

2. Enable Compression

CloudFront can automatically compress objects using Gzip and Brotli, dramatically reducing transfer size for text-based assets (HTML, CSS, JS, JSON).

• Set Compress: true in your cache behavior.
• Ensure the viewer sends Accept-Encoding: gzip, br.
• Only objects between 1,000 and 10,000,000 bytes are compressed.

Brotli typically achieves 15–25% better compression than Gzip for text content.

3. Use Origin Shield

Origin Shield adds an additional caching layer between edge locations and your origin. All edge caches funnel misses through a single regional cache, which collapses duplicate requests and further protects your origin.

Enable it in the region closest to your origin:

Origin Shield Region: us-east-1

This is especially valuable for origins with limited capacity or high request volumes.

4. Optimize with Cache and Origin Request Policies

Separate what's used for the cache key from what's forwarded to the origin. You might not want to vary the cache by an Authorization header but still need to forward it to the origin. Use an Origin Request Policy for this.

aws cloudfront create-origin-request-policy \
  --origin-request-policy-config file://origin-request-policy.json

5. Leverage Edge Functions

Move logic to the edge to reduce origin load and latency:

Example CloudFront Function to add security headers:

function handler(event) {
  var response = event.response;
  response.headers['strict-transport-security'] = {
    value: 'max-age=63072000; includeSubDomains; preload'
  };
  return response;
}

6. Control Costs

• Choose the right price class. If your audience is regional, use PriceClass_100 (North America and Europe only) to avoid paying for expensive edge locations.
• Monitor data transfer. Origin-to-edge transfer over CloudFront is often cheaper than direct S3 or EC2 egress.
• Use Savings Bundles if you have predictable, high-volume traffic.

7. Monitor and Iterate

Enable real-time logs and CloudWatch metrics to track:

• CacheHitRate — aim for 85%+ on static content
• OriginLatency — spikes indicate origin bottlenecks
• 4xxErrorRate / 5xxErrorRate — configuration or origin issues

aws cloudwatch get-metric-statistics \
  --namespace AWS/CloudFront \
  --metric-name CacheHitRate \
  --dimensions Name=DistributionId,Value=EXXXXXX \
  --statistics Average --period 3600

Conclusion

CloudFront optimization is an iterative process. Start by normalizing cache keys and setting sensible TTLs to boost your hit ratio, enable compression and Origin Shield to reduce origin load, and push logic to the edge with functions. Continuously monitor your metrics and refine your policies. These changes compound into faster user experiences and meaningful cost savings.