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CDN / Cloud

Cache hits, edge responses and cloud configuration.

The tools in the CDN / Cloud category help you quickly diagnose the caching, expiry, and freshness problems that come up when you run a content delivery network or edge infrastructure. They answer questions like: why is my CDN cache hit ratio (HIT vs MISS) low, is the edge actually returning the response headers I expect, will my presigned or signed URL expire at the exact moment I think once clock skew is accounted for, and how should I set max-age alongside stale-while-revalidate so users always get a fast response while origin load stays low. They're aimed at frontend and backend developers who own web performance, DevOps and SRE engineers, and anyone operating services like Cloudflare, CloudFront, Fastly, or S3.

A typical workflow starts by looking at the real response headers. You read Cache-Control, Age, X-Cache, and ETag to judge whether content is being served from cache and whether the freshness window is right, then use a stale-while-revalidate planner to dial in the max-age and SWR values if it isn't. If you rely on signed URLs, computing the issue time, lifetime, and server-to-CDN clock skew together lets you head off the dreaded sudden 403. And if bandwidth cost is a concern, estimating the monthly CDN bill from transfer volume helps you weigh the cost impact of your caching policy. Every calculation runs entirely in your browser, so the values you enter are never sent anywhere.

Stale-While-Revalidate Planner
Plan max-age and stale-while-revalidate values and output the header.
Signed URL Expiry Calculator
Compute signed URL expiry considering duration and clock skew.

Frequently asked questions

How do I check whether my CDN cache is hitting?
Look at the response headers. CloudFront reports X-Cache: Hit from cloudfront, Cloudflare uses cf-cache-status: HIT, and Fastly returns X-Cache: HIT, so each provider tells you the cache status in a header. An Age header greater than 0 also means the response came from cache. The surest test is to request the same URL twice and confirm it goes from MISS on the first hit to HIT on subsequent ones.
What's the difference between max-age and stale-while-revalidate?
max-age is how long a response is considered fresh and reused as-is. stale-while-revalidate is an additional window after max-age expires during which the cache immediately serves the stale copy while fetching a fresh one in the background. Used together, users always get a fast response and your origin avoids a thundering herd of simultaneous revalidation requests.
Why does my signed URL expire earlier than expected?
Usually it's clock skew. If the server that issued the URL and the CDN or storage that validates it have even slightly different clocks, the effective lifetime can be shorter than intended. It's also common to mistake the expires value's unit (seconds vs minutes) or to confuse UTC with local time. Calculating the issue time, lifetime, and allowed skew together helps you avoid sudden 403 expiry errors.
Do the tools in this category connect directly to my server to test it?
Most of them are calculators and planners that run entirely in your browser. The header values, URL lifetimes, and traffic figures you enter are processed on your device and never sent anywhere. That means they add no load to your live service, and it's safe to enter sensitive values while you check things.
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