CDN: what it is and why you need it

What is a CDN

A CDN (Content Delivery Network) is a network of servers (nodes, or PoPs — Points of Presence) distributed geographically around the world. When a user requests a resource, the CDN serves it from the server closest to the user instead of from the website’s origin server.

A CDN significantly reduces latency and TTFB, especially for users in other countries.

How a CDN works

Без CDN:
Пользователь (Москва) ────────────────────── Сервер (Франкфурт)
                              ~30 мс RTT

С CDN:
Пользователь (Москва) ── CDN Edge (Москва) ── Origin (Франкфурт)
                    ~3 мс RTT    кеш файлов

Повторный запрос:
Пользователь (Москва) ── CDN Edge (Москва)
                    ~3 мс RTT  (без обращения к origin)

What CDNs cache

CDNs typically cache:

• Static files — CSS, JS, images, fonts
• Video and audio — streaming media
• Static HTML pages — SSG content
• API responses — when caching is configured

CDNs do not cache by default:

• Dynamic content (which depends on the user)
• POST requests
• Data for authenticated users

Popular CDN services

Setting up Cloudflare CDN

1. Sign up at cloudflare.com
2. Add your domain
3. Change your domain’s NS records to Cloudflare’s name servers
4. Configure Page Rules for caching

// Заголовки для кеширования статики
Cache-Control: public, max-age=31536000, immutable
// для версионированных файлов (bundle.abc123.js)

CDN and security

Cloudflare and other CDNs provide additional protection:

• DDoS protection — filtering malicious traffic
• WAF (Web Application Firewall) — blocking attacks
• Bot management — protection from scrapers
• SSL/TLS — free certificates and management

CDN cache invalidation

When you update files, you need to clear the cache:

# Cloudflare API
curl -X POST "https://api.cloudflare.com/client/v4/zones/ZONE_ID/purge_cache" \
  -H "Authorization: Bearer TOKEN" \
  -d '{"purge_everything": true}'

Or use file versioning: bundle.v123.js — when a file changes, its name changes, so the old cache does not get in the way.

Checking HTTP/2 on reChecker

Use HTTP/2 & Brotli to check the protocol and compression on your website. Many CDNs enable HTTP/2 and Brotli automatically — this tool will confirm that they are working.

FAQ

Is a CDN essential for a small website? It is not essential, but it is useful. Even Cloudflare’s free plan speeds up your site and provides DDoS protection and free SSL — benefits worth using for any website.

Does a CDN hurt SEO? No. A CDN improves SEO by reducing TTFB and improving Core Web Vitals. Google considers loading speed a ranking factor.

What are “cache miss” and “cache hit”? A cache hit means the request was served from the CDN cache (fast). A cache miss means the CDN does not have the item cached, so the request goes to the origin server (slower). A high cache hit rate (90%+) is a CDN optimization goal.