CloudFront Caching Optimization: How to Maximize Performance

Key Takeaway

The key to CloudFront performance optimization is increasing the cache hit ratio. Reduce origin load and improve user response time with proper TTL settings, minimized cache keys, compression enablement, and Origin Shield usage.

How Does CloudFront Work?

CloudFront caches content at 450+ edge locations worldwide, responding from the location closest to the user.

[User Request]
     │
     ▼
┌────────────────┐
│ Edge Location  │ ─── Cache HIT → Immediate response (fast)
│ (Seoul)        │
└────────────────┘
     │
     │ Cache MISS
     ▼
┌────────────────┐
│ Origin Shield  │ ─── Cache HIT → Protects origin
│ (Optional)     │
└────────────────┘
     │
     │ Cache MISS
     ▼
┌────────────────┐
│ Origin         │ ─── S3, ALB, EC2, etc.
│ (Origin Server)│
└────────────────┘

Why Cache Hit Ratio Matters

Cache Hit Ratio	Effect
High (90%+)	Reduced origin load, cost savings, fast responses
Low (below 50%)	Origin overload, high costs, slow responses

Understanding Cache Policies

Cache policies define what values to include in the cache key and TTL settings.

What is a Cache Key?

The cache key is the identifier CloudFront uses to look up requests in the cache.

Default Cache Key: URL path + Query string

Example:
/images/logo.png → Cache Key A
/images/logo.png?v=1 → Cache Key B (treated as different object!)

Why Minimizing Cache Key Matters

The more values included in the cache key, the lower your cache hit ratio.

Included in Cache Key	Result
All headers	❌ Very low hit ratio
All cookies	❌ Different cache per user
Unnecessary query strings	❌ Same content cached differently
Minimal values	✅ High hit ratio

Managed Cache Policies

AWS provides managed cache policies for common use cases:

Policy Name	Use Case	TTL
CachingOptimized	Static content (recommended)	Default 24 hours, max 1 year
CachingDisabled	Disable caching	0
Elemental-MediaPackage	Media streaming	-

TTL (Time To Live) Optimization

TTL determines how long a cached object remains valid.

TTL Setting Priority

1. Origin's Cache-Control/Expires headers
2. CloudFront cache policy Min/Max/Default TTL

Recommended TTL by Content Type

Content Type	Recommended TTL	Reason
Static assets (JS, CSS, images)	1 year (31536000 sec)	When filename includes hash
HTML pages	5 min to 1 hour	May change frequently
API responses	0-60 sec	Real-time requirements
Video/Audio	1 day to 1 week	Large size, high caching benefit

Cache-Control Header Settings

You can control TTL directly from the origin:

# 1 year caching (static assets)
Cache-Control: max-age=31536000, public

# No caching (dynamic content)
Cache-Control: no-cache, no-store, must-revalidate

# 5 minute caching + revalidation
Cache-Control: max-age=300, must-revalidate

Cache Invalidation

A method to forcibly refresh the cache when content changes.

Invalidation Methods

# Invalidate via AWS CLI
aws cloudfront create-invalidation \
    --distribution-id E1234567890ABC \
    --paths "/images/*" "/css/*"

Invalidation Costs

Item	Cost
First 1,000 paths/month	Free
Per additional path	$0.005

Invalidation vs Versioning

Method	Pros	Cons
Invalidation	Immediate effect	Costs, propagation time
Versioning	Immediate effect, no cost	URL change required

Versioning Example:
/css/style.css → /css/style.abc123.css (include hash)

Enabling Compression

CloudFront supports Gzip and Brotli compression.

Compression Benefits

File Type	Original Size	Gzip	Brotli
JavaScript	100KB	30KB	25KB
CSS	50KB	12KB	10KB
HTML	80KB	20KB	16KB

Compression Configuration

Enable compression in cache behavior:

1. CloudFront Console → Distribution → Edit Behavior
2. Enable Compress objects automatically
3. Enable EnableAcceptEncodingGzip, EnableAcceptEncodingBrotli in cache policy

Leveraging Origin Shield

Origin Shield is an additional caching layer that further reduces origin load.

Standard Configuration:
[User] → [Edge Location] → [Origin] (many requests)

Origin Shield Configuration:
[User] → [Edge Location] → [Origin Shield] → [Origin] (fewer requests)

Origin Shield Selection Criteria

Scenario	Origin Shield Recommendation
Global users + single origin	✅ Strongly recommended
High origin costs (EC2, Lambda)	✅ Recommended
Already Multi-Region origins	⚠️ Optional

Origin Shield Region Selection

Choose the region closest to your origin:

• S3 bucket in ap-northeast-2 → Origin Shield in ap-northeast-2
• ALB in us-east-1 → Origin Shield in us-east-1

SAA-C03 Exam Focus Points

1. ✅ Improve cache hit ratio: "Minimize cache key, exclude unnecessary headers/cookies"
2. ✅ TTL strategy: "Long TTL for static assets + versioning"
3. ✅ Origin Shield: "Reduce origin load, for global users"
4. ✅ Compression: "Enable Gzip/Brotli for text-based content"
5. ✅ Invalidation: "Use for urgent updates, has cost"

Frequently Asked Questions

Q: What's an appropriate cache hit ratio?

Target 90%+ for static content, 70-80% when mixed with dynamic content. Monitor with CloudWatch's CacheHitRate metric.

Q: Is invalidation applied immediately?

After submitting an invalidation request, it typically takes a few minutes to propagate to all edge locations worldwide. Use only for emergencies; versioning is recommended for regular updates.

Q: What happens when S3 and CloudFront cache settings conflict?

CloudFront respects the origin's (S3) Cache-Control headers. However, they only apply within the CloudFront cache policy's Min/Max TTL range.

Q: Does Origin Shield incur additional costs?

Yes. Additional fees apply for requests processed through Origin Shield. However, reduced origin requests may result in overall cost savings.

Q: What's the difference between Lambda@Edge and CloudFront Functions?

CloudFront Functions are suitable for lightweight operations (header manipulation) at viewer request/response. Lambda@Edge is suitable for complex logic at origin request/response.

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Related Posts

• S3 Storage Classes Complete Guide
• Route 53 Routing Policies
• VPC Endpoints: Interface vs Gateway

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References

• CloudFront Cache Policy Understanding
• Using Managed Cache Policies
• CloudFront Origin Shield
• CloudFront Caching Best Practices