DNS, HTTPS, CDN and Caching
Explain why a fast site can sometimes show old information. · Technical Fluency · Lesson 25 · 3 min
DNS, HTTPS, CDN and Caching · 3 min
Situation
The update is live, but someone sees the old page.
The team deploys a correction to a static site. One reader sees it immediately; another still sees the old version.
The request may pass through browser and network caches. “We deployed it” and “every client is showing it” are different observations.
Process
Follow the address to the content.
For an illustrative domain such as productfluency.com, DNS helps locate the destination. HTTPS protects the connection using encryption and certificate-based server authentication. A CDN can serve content from locations closer to users or forward requests to an origin server.
These are different jobs. HTTPS does not guarantee the accuracy of the page, and DNS does not store the page itself.
Mental model
A cache reuses an earlier result.
Caching avoids repeating work and can reduce latency, bandwidth, and origin load. It also requires rules about how long a result may be reused and when it should be refreshed.
Static assets with versioned filenames are easier to cache for long periods. Frequently changing account data needs a more careful freshness and privacy policy.
Failure case
The wrong cache key exposes the wrong result.
If a personalized response is cached as though it were public, one user's data could be served to another. If a price response is reused too long, users may see stale prices.
Ask which responses are safe to cache, what identifies distinct results, and which changes require invalidation or revalidation.
PM decision
Define acceptable staleness.
A public help article might tolerate a short delay. A permission change or final checkout price may require stronger freshness guarantees.
For a reported stale page, capture the URL, time, device, and response behavior. Compare the origin and cached paths before assuming the deployment failed. Do not ask every customer to clear all browser data as the primary fix.
Remember this
Faster delivery comes with freshness decisions.
State how current each kind of information must be and who may see it. That gives engineering a product requirement for caching behavior.