CloudFront keeps GETting pages from your website because it is caching responses based on query strings, cookies, or headers that you haven't explicitly configured to vary. This behavior often leads to stale content being served to users or, conversely, excessive origin traffic if every unique request bypasses the cache. The core issue lies in the mismatch between how CloudFront constructs cache keys and how your application generates dynamic URLs.
Understanding Cache Key Construction
When a user requests a page, CloudFront creates a unique identifier for that response called a cache key. By default, this key includes the URL path and query string parameters. If your website uses query strings to track analytics, session IDs, or A/B testing variants, CloudFront treats each variation as a separate object. Consequently, it sends a request to your origin server for every new combination of parameters, even if the visual content is identical. This results in the "GETting" behavior where the origin receives numerous requests for what appears to be the same page. To resolve this, you must review the distribution settings under the
Behaviors tab. Look for the option labeled
Query String Forwarding and Caching. Setting this to
None ensures that only the path determines the cache entry, ignoring tracking parameters entirely.
Handling Cookies and Headers
Similarly, cookies can inadvertently trigger cache misses. If your site sets session cookies or authentication tokens, and CloudFront is configured to forward all cookies to the origin, it will treat every user's session as a unique cache entry. This not only causes the origin to process more requests but also prevents content sharing across users, defeating the purpose of a CDN. Navigate to the
Cache Keys and Origin Requests section in your CloudFront console. Here, you can specify exactly which cookies should be forwarded. Typically, you should forward only essential cookies like
session_id or
auth_token, while discarding analytics or advertising cookies. This reduces the number of unique cache objects and lowers origin load significantly.
Troubleshooting Stale Content Issues
While reducing cache misses is crucial, another common problem is serving outdated content. If you update a file on your website, CloudFront may continue to serve the old version from its edge locations until the Time to Live (TTL) expires. This happens because CloudFront respects the
Cache-Control headers sent by your origin. If your origin does not set appropriate TTLs, CloudFront defaults to conservative values, such as 24 hours. To fix this, ensure your web server returns accurate
max-age directives in the
Cache-Control header. For static assets like CSS and JavaScript, you can set long TTLs (e.g., one year) combined with versioned filenames. For dynamic HTML pages, shorter TTLs (e.g., 5 minutes) are more appropriate. You can override these settings directly in CloudFront by creating a custom cache policy that ignores origin headers and enforces your own TTL rules.
Invalidating the Cache Manually
Sometimes, immediate updates are required without waiting for TTL expiration. In these cases, you can manually invalidate specific paths. Go to the
Distributions dashboard, select your distribution, and click the
Invalidations tab. Create a new invalidation and enter the paths you want to purge, using wildcards like
/* for broad changes. Note that invalidations incur additional costs and take time to propagate globally, so they should be used sparingly for critical updates rather than as a routine deployment strategy.
Monitoring and Optimization
To verify your configuration, monitor the
Metrics tab in the CloudFront console. Look at the
CacheHitRate metric. A low hit rate indicates that too many requests are reaching your origin, confirming that your cache key configuration is too granular. Additionally, check the
OriginResponseTime to ensure your backend is handling the reduced load efficiently. If you notice spikes in traffic after deploying new features, revisit your cache policies to ensure no new query parameters or cookies have been introduced that break caching. For complex scenarios involving multiple origins or regional variations, consider using Lambda@Edge functions to manipulate requests before they reach the cache lookup stage. This allows for sophisticated logic, such as stripping specific tracking parameters dynamically based on user agents or referrers. By aligning your cache keys with your actual content needs, you eliminate unnecessary GET requests to your origin and improve overall site performance.
For more information on managing website resources effectively, see
Downloading Links to Multiple Website Documents and
Troubleshooting: Website no longer displaying unicode properly.
Frequently Asked Questions
Why is my CloudFront cache miss rate high?
A high cache miss rate usually means your cache keys are too unique, often due to forwarding unnecessary cookies or query strings. Review your cache policy settings and restrict forwarded items to only those strictly required for functionality.
How do I prevent CloudFront from caching dynamic API responses?
Set the TTL to zero for API endpoints or use a custom cache policy that forwards all necessary headers but caches nothing. Alternatively, configure your origin to send
Cache-Control: no-store headers for dynamic data.
Can I clear the entire CloudFront cache instantly?
You can create an invalidation with the wildcard pattern
/* to purge all cached objects. However, this action takes time to propagate and incurs fees, so it should be used only when absolutely necessary.
What happens if I change the cache key settings after traffic has built up?
Existing cached objects remain valid until their TTL expires. New requests will generate new cache entries based on the updated key, potentially leading to temporary duplicate storage until old entries expire or are invalidated.
Does CloudFront support gzip compression automatically?
Yes, CloudFront automatically compresses certain text-based content types like HTML, CSS, and JSON if the client supports it. You do not need to configure compression manually unless you are dealing with non-standard MIME types.