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CDN (Content Delivery Network)

Cache content at edge nodes close to users — the foundation of modern latency, bandwidth, and DDoS defense.

CDNEdgeCaching
核心 · Key Idea

In one line: a CDN caches static assets at hundreds of edge points of presence worldwide; users hit the nearest one. The origin only sees cache-miss "fill" traffic, capacity grows by orders of magnitude, and intercontinental access drops from seconds to milliseconds.

What it is#

User → nearest edge PoP
        cache hit  → return immediately
        cache miss → pull from origin → cache → return

DNS routes the user to "the nearest edge IP" via Anycast + GeoDNS.

Analogy#

打个比方 · Analogy

The origin is a central warehouse; CDN edges are convenience stores everywhere. Hot products (cache hits) you buy at the corner store; cold ones (misses) the store fetches from the warehouse for you and stocks the shelf for next time.

Key concepts#

OriginOrigin
Your real server. CDN visits it on cache miss.
Edge / PoPEdge / Point of Presence
Globally distributed cache servers.
TTLCache TTL
Driven by response headers Cache-Control / Expires.
Cache KeyCache Key
Decides which requests share the same cache entry — usually URL; can include query / cookie.
PurgePurge
Force the CDN to drop a stale entry and re-fetch from origin.
AnycastAnycast
Multiple PoPs announce the same IP; BGP steers users to the closest one.

How it works#

The origin only sees N misses while the system absorbs hundreds-to-thousands × the request volume.

Practical notes#

  • Set Cache-Control correctly:

    Cache-Control: public, max-age=31536000, immutable    # static assets
Cache-Control: no-store                               # user-private data
Cache-Control: public, max-age=60, s-maxage=600       # pages: browser 1m, CDN 10m

  • Use versioned filenames (/app.abc123.js): a new release just bumps the hash and the CDN naturally rotates.

  • Don't overuse Vary: Vary: Cookie shards the cache per-cookie — hit rate collapses.

  • Run HTTPS end-to-end: set origin to HTTPS too; avoid "Flexible SSL"-style cleartext-to-origin modes.

  • Edge compute: Cloudflare Workers / AWS Lambda@Edge can mutate requests/responses without going back to origin.

Easy confusions#

CDN
**Many** nodes worldwide.
Mostly caching + acceleration + DDoS absorption.
Reverse proxy (nginx)
Usually **one** front-proxy.
Routing / auth / canary / TLS termination.

Further reading#