Glossary Term
CDN (Content Delivery Network)
A CDN is a network of edge servers that cache and serve responses near users. See how cache hits show up in headers and what CDNs do not cache by default.
On this page
TL;DR: A CDN is a fleet of reverse proxies spread across regions that cache your responses and serve them from the nearest edge, so most requests never reach your origin server.
A content delivery network (CDN) is a geographically distributed set of caching reverse proxies. Browsers connect to a nearby edge server; on a cache hit the edge answers directly, and on a miss it fetches from your origin, stores the response according to its caching headers, and returns it.
What a hit looks like
HTTP/1.1 200 OK
Content-Type: text/css
Cache-Control: public, max-age=31536000, immutable
Age: 4312
CF-Cache-Status: HITAge is the number of seconds the response has sat in a shared cache, as defined by RFC 9111. The status header is vendor-specific: Cloudflare uses CF-Cache-Status, CloudFront uses X-Cache: Hit from cloudfront plus X-Amz-Cf-Id, and other vendors use their own X-Cache variants. No Age on a response you expected to be cached usually means a miss or a bypass.
Non-obvious facts
- A CDN follows your caching headers, then its own rules.
Cache-Control: s-maxageapplies to shared caches such as CDNs only, so you can give the edge an hour while browsers revalidate every time.privateandno-storekeep a response off the edge. - HTML is often not cached by default. Cloudflare, for example, caches by file extension and does not cache HTML unless a cache rule says so. If pages are slow, check this before blaming the network.
Varyfragments the cache.Vary: Accept-Encodingis fine;Vary: User-AgentorVary: Cookiecan drop the hit ratio to near zero because every distinct value gets its own entry.- A CDN adds another place to debug. A 502 or 504 may come from the edge, not your server. Compare headers such as
Server,ViaandCF-Raywith a request made directly to the origin. - Purging is not instant everywhere. Invalidations take time to propagate across the network, so versioned asset URLs are more reliable than purging.
Go deeper
Frequently asked questions
What does a CDN do?
It terminates connections close to the user and serves cacheable responses from edge servers, falling back to your origin on a miss. It also absorbs traffic spikes and TLS work.
How can I tell if a response came from the CDN cache?
Look for Age greater than zero and a vendor header such as CF-Cache-Status: HIT on Cloudflare or X-Cache: Hit from cloudfront on CloudFront.
Does a CDN cache HTML?
It depends on configuration. Many CDNs cache static file extensions by default and pass HTML through unless Cache-Control or a rule says otherwise.
Is a CDN the same as a reverse proxy?
A CDN is a reverse proxy that is geographically distributed and caches. Every CDN edge is a reverse proxy; not every reverse proxy is a CDN.
Sources
Related
X-Cache Header
Learn how the X-Cache header indicates cache hit or miss status from CDNs and proxies. Debug caching issues and verify CDN configuration with this header.
Cache-Control Header: Directives, Examples and CDN Behavior
Cache-Control directives explained: max-age, no-cache vs no-store, s-maxage, stale-while-revalidate, immutable, with nginx, Cloudflare and Next.js examples.
Age Header
Learn how the Age header indicates how long a response has been cached in seconds. Understand cache freshness calculations and CDN behavior.
Reverse Proxy
A reverse proxy sits in front of origin servers and forwards client requests to them. Learn the headers it adds, nginx proxy_pass pitfalls and 502/504 causes.