Serve static and semi-static content from edge servers physically close to the user.
Request a file. The first trip goes to the far origin; the edge caches it.
A CDN keeps copies of your static files on edge servers around the world. A user in Tokyo hits a nearby edge instead of your origin in Virginia, so after the first request the file loads in milliseconds. Closer equals faster.
Plain English: copy your images and videos onto servers in cities all over the world, so a user in Tokyo doesn't wait for the file to come from your data center in Virginia. Closer = faster.
A global network of edge servers that cache content from your origin and serve it to nearby users. Same content, but served from milliseconds away instead of half a continent.
The speed of light is slow when your users are 8,000 km from your origin. CDNs put a cached copy of your content within ~50ms of any user on the planet. They also offload large amounts of bandwidth from your origin, which cuts cost.
User requests an asset (image, video chunk, JS bundle). DNS routes them to the nearest CDN POP. If the POP has the asset cached, it serves immediately. If not, it fetches from origin, caches locally with a TTL, and serves. Subsequent requests in that region hit the cache.
Photos and videos served from CDN edge; origin is S3
Open Connect, Netflix's own CDN, with appliances placed inside ISPs
Video chunks served from CDN; client prefetches the next 2-3 videos in parallel
A customer config trigger crashed Fastly globally: 49 minutes, half the modern web dark.
Fastly had pushed a config update weeks earlier that introduced a latent bug, only triggered by a specific customer configuration pattern. When that customer eventually applied their config, the bug fired across Fastly's global edge fleet within 12 seconds. Reddit, the NYT, Amazon, the UK Gov website: all 503ing simultaneously. Recovery took 49 minutes because the rollback procedure itself depended on healthy edge nodes. The lesson: latent bugs triggered by customer input are essentially production bombs. Canary deployments must rotate, and your incident-response paths must work even when your data plane is on fire.
A replication topology change made Workers KV return data that was hours old globally.
Cloudflare Workers KV is a globally distributed key-value store built on eventual consistency: writes propagate to all edges within roughly 60 seconds. During a maintenance operation, an engineer changed the replication topology, specifically which nodes a region's reads fall back to on cache miss. The change accidentally routed reads for a subset of keys to a secondary tier that had stopped receiving updates. Edge nodes across all regions started serving stale values that were hours old, not seconds old. Feature flags, A/B test configs, and auth tokens stored in KV returned wrong results for 35 minutes. The lesson: eventually consistent systems have a defined propagation bound. Any change to the replication topology must be validated against that bound. Breaking propagation doesn't produce errors; it produces silent staleness that can persist indefinitely.
CDNs are expected in any system serving media or global users, so the signal isn't proposing one but explaining what it actually buys you. Say it reduces latency by moving bytes physically closer to the user and offloads origin traffic. The nuance to show is around cache-control headers and invalidation: a misconfigured TTL means stale content globally and you can't easily fix it. Candidates lose points by treating the CDN as a magic fix for all latency without addressing what happens when the edge cache is stale or cold.