Glossary
Content delivery network
A content delivery network is a set of servers distributed geographically that cache a site's content and serve it from whichever location is closest to each visitor. It reduces latency by shortening the physical distance a request travels, absorbs traffic that would otherwise reach the origin, and usually provides TLS termination, compression and protection against volumetric attacks along the way. What it cannot do is make a slow application fast for responses that cannot be cached.
What it actually improves
Distance. A visitor in one continent fetching from an origin in another pays that round trip on every request; an edge cache a few hundred kilometres away does not.
Origin load. Requests served from cache never reach the origin, which is what lets a modest server handle a traffic spike.
Connection setup. The TLS handshake terminates at the nearby edge, so the expensive part of connection establishment is local even when the content is not cached.
Availability during origin trouble, where a well-configured edge can serve stale content rather than an error.
What it cannot improve
Uncacheable responses still go to the origin. A personalised page is generated there, and the CDN adds a hop rather than removing one — though the shortened TLS setup usually still nets a gain.
Server processing time. A query taking two seconds takes two seconds whether the request arrived directly or through an edge.
Front-end cost. A page with three megabytes of JavaScript is slow everywhere, and delivering it faster does not change how long it takes to execute.
Correctness. A CDN caching a personalised page and serving it to the wrong visitor is a data leak, and it is an easy misconfiguration to produce.
What changes when one is in front
Headers may be added, rewritten or stripped at the edge, so the origin's configuration is no longer what the visitor receives.
The address a hostname resolves to belongs to the CDN, which is why an ASN lookup names the CDN rather than the hosting provider.
The origin remains reachable directly unless it is restricted to the CDN's addresses, and an exposed origin allows the whole edge layer to be bypassed.
Cache behaviour becomes a configuration surface of its own: what is cached, keyed on what, for how long, and what invalidates it.
How to see whether one is in use
Response headers usually name it: a `server` value, a vendor-specific identifier, or a cache status header reporting a hit or a miss.
The ASN of the resolved address identifies the network serving the request.
A cache status header reporting a miss on every request means the CDN is present and caching nothing, which is a configuration issue rather than an absence.
Compare the response from the public hostname against one from the origin hostname, where you know it. Differences identify what the edge is adding.
Frequently asked questions
- Does a CDN make every site faster?
- It helps most for geographically distributed visitors and cacheable content. For a local audience and fully dynamic responses the gain is small.
- Why does my cache status always say miss?
- The responses are not being cached, usually because of the cache-control headers the origin sends or a cache key that includes something unique per request.
- Can visitors bypass the CDN?
- Yes, if the origin server remains reachable at its own address. Restricting the origin to the CDN's addresses is what prevents it.
Sources
Related
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