Glossary · Performance

CDN (Content Delivery Network)

A CDN is a geographically distributed network of servers that caches your content close to visitors, reducing latency and shielding your origin from load.

Updated October 5, 2026

A content delivery network (CDN) is a network of edge servers in many locations that cache and serve your content from a point near the visitor. Requests the edge can answer never reach your origin server.

Why it matters

A CDN cuts round-trip latency for distant visitors, reduces origin bandwidth and CPU use, and can absorb traffic spikes and some attacks. It can also keep serving cached pages when your origin is briefly down, depending on how it is configured. The cost is another layer that can misbehave: stale content, wrong cache keys and origin connection errors.

Example

Response headers reveal how the edge handled a request. Header names differ by provider, but the pattern is similar:

Cache-Control: public, max-age=3600, s-maxage=86400
Age: 1240
X-Cache: HIT

s-maxage sets how long shared caches such as a CDN may keep the object, while max-age governs browsers. Age shows how long it has been in the edge cache.

Common mistakes

  • Caching responses that vary by cookie or login state, and serving one user's page to another.
  • Setting long cache lifetimes with no purge or versioned file names, so updates do not appear.
  • Leaving the origin reachable directly, which bypasses the CDN and its protections.
  • Monitoring only the CDN edge. If the edge serves cached copies, an origin failure can go unnoticed for hours.

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