Amazon CloudFront is a content delivery network (CDN) that caches content at edge locations worldwide to reduce latency and improve performance. Understanding cache behavior is essential for troubleshooting and optimization.
**Cache Key Components:**
CloudFront generates cache keys based on the re…Amazon CloudFront is a content delivery network (CDN) that caches content at edge locations worldwide to reduce latency and improve performance. Understanding cache behavior is essential for troubleshooting and optimization.
**Cache Key Components:**
CloudFront generates cache keys based on the request URL, headers, query strings, and cookies you configure. By default, only the URL path is used. Adding more components increases cache granularity but may reduce cache hit ratios.
**TTL (Time To Live):**
CloudFront uses three TTL settings: Minimum TTL, Maximum TTL, and Default TTL. These work alongside origin Cache-Control headers. If your origin sends Cache-Control: max-age, CloudFront respects it within your configured bounds.
**Cache Hit Ratio Optimization:**
To improve cache hit ratios, minimize unnecessary query string parameters, normalize headers, and use consistent URL patterns. Monitor CloudFront metrics like CacheHitRate in CloudWatch to identify optimization opportunities.
**Cache Behaviors:**
You can configure multiple cache behaviors per distribution, each matching specific path patterns. Each behavior can have different origins, TTL settings, and allowed HTTP methods. Behaviors are evaluated in order, with the default behavior (*) as a fallback.
**Cache Invalidation:**
When content changes, you can invalidate cached objects using the InvalidatePath API or console. Invalidations typically complete within 60 seconds but incur costs beyond 1,000 free paths monthly. Using versioned file names is often more cost-effective.
**Troubleshooting Headers:**
CloudFront adds response headers like X-Cache (Hit/Miss from cloudfront) and Age (seconds since cached) to help diagnose caching issues. Enable access logs for detailed request analysis.
**Common Issues:**
Low cache hit ratios often result from forwarding unnecessary cookies or query strings, varying Accept-Encoding headers, or short TTLs. Review your cache policy configuration and origin responses to optimize performance.
CloudFront cache behaviors are fundamental to optimizing content delivery and controlling how CloudFront handles requests. Understanding cache behaviors is essential for the AWS Developer Associate exam because they directly impact application performance, cost optimization, and security implementation. Misconfigurations can lead to serving stale content, security vulnerabilities, or unnecessary origin load.
What is CloudFront Cache Behavior?
A cache behavior is a set of rules that tells CloudFront how to handle requests for specific URL patterns. Each CloudFront distribution has a default cache behavior that applies to all requests, and you can create additional precedence-based cache behaviors for specific path patterns.
Key components of cache behavior include: - Path Pattern: URL pattern matching (e.g., /images/*, /api/*, *.jpg) - Origin: Where CloudFront forwards requests when cache misses occur - Viewer Protocol Policy: HTTP/HTTPS handling - Cache Policy: TTL settings and cache key configuration - Origin Request Policy: What headers, cookies, and query strings to forward
How CloudFront Cache Behavior Works
Request Flow: 1. User makes a request to CloudFront 2. CloudFront evaluates path patterns in precedence order (0 being highest) 3. The first matching cache behavior is applied 4. If no pattern matches, the default cache behavior is used 5. CloudFront checks if the object is cached based on the cache key 6. On cache hit, content is served from edge location 7. On cache miss, request goes to origin based on behavior settings
TTL Settings: - Minimum TTL: Shortest time objects stay cached - Maximum TTL: Longest time objects stay cached - Default TTL: Applied when origin doesn't specify Cache-Control headers
Viewer Protocol Policy Options: - HTTP and HTTPS: Accept both protocols - Redirect HTTP to HTTPS: Automatically redirect insecure requests - HTTPS Only: Reject HTTP requests
Origin Protocol Policy Options: - HTTP Only: CloudFront connects to origin via HTTP - HTTPS Only: CloudFront connects to origin via HTTPS - Match Viewer: Uses the same protocol as the viewer request
Common Cache Behavior Patterns
Static Content: Long TTL, minimal forwarded headers Dynamic Content: Short or zero TTL, forward necessary headers/cookies API Endpoints: Often bypass cache, forward all headers and query strings
Exam Tips: Answering Questions on CloudFront Cache Behavior
1. Path Pattern Precedence: Remember that cache behaviors are evaluated in order of precedence. The default behavior (precedence *) is always evaluated last. Questions may test your understanding of which behavior applies to a given URL.
2. Cache Invalidation vs. Versioning: When asked about updating cached content, versioning (changing file names) is more cost-effective than invalidation. Invalidation has costs and takes time to propagate.
3. Query String Handling: If the exam mentions that different query parameters should serve different content, you need to configure the cache behavior to forward and cache based on query strings.
4. Cookie Forwarding: For personalized content or session-based applications, forwarding cookies is necessary but reduces cache hit ratio. Know when to forward all, none, or specific cookies.
5. Header Forwarding: Forwarding headers like Accept-Language for localized content creates separate cache entries. Be aware this affects cache efficiency.
6. Origin Failover: For high availability questions, remember that origin groups allow automatic failover to a secondary origin when the primary fails.
7. Signed URLs vs Signed Cookies: For restricting access, signed URLs work for individual files while signed cookies are better for multiple restricted files.
8. TTL Scenarios: When origin Cache-Control headers conflict with CloudFront TTL settings, CloudFront uses the more restrictive value between origin headers and Maximum TTL.
9. Lambda@Edge Integration: Cache behaviors can trigger Lambda@Edge functions at four points: viewer request, origin request, origin response, and viewer response.
10. Troubleshooting Stale Content: If questions describe serving outdated content, consider: cache TTL settings, missing invalidations, or incorrect cache key configuration.