CDN in 5 Minutes
Real Incident: Reddit Hug of Death
A blog goes viral — origin server handles 50 req/s normally, Reddit sends 50,000 req/s. Server dies in 30 seconds. With CDN: 95% served from edge, origin sees 2,500 req/s. CDN is the difference between going viral and going down.
The One-Liner
A CDN caches your content on servers spread worldwide so users get data from a nearby location instead of your distant origin server.
How It Works
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flowchart LR
U1[User Tokyo] --> E1[Edge Tokyo]
U2[User London] --> E2[Edge London]
U3[User NYC] --> E3[Edge NYC]
E1 -.->|cache miss| O[Origin Virginia]
E2 -.->|cache miss| O
E3 -.->|cache miss| O
style O fill:#ef4444,color:#fff
style E1 fill:#22c55e,color:#fff
style E2 fill:#22c55e,color:#fff
style E3 fill:#22c55e,color:#fff- Without CDN: Tokyo user → 12,000km to Virginia origin → 200ms latency
- With CDN: Tokyo user → Tokyo edge (5km) → 5ms latency
- Physics: light in fiber ≈ 200,000 km/s. No engineering beats physics. CDN puts content close.
- Typical cache hit rate: 95%+ — origin only handles 5% of traffic
Push vs Pull
| Aspect | Pull (Origin-Pull) | Push (Origin-Push) |
|---|---|---|
| How | Edge fetches on cache miss | You upload proactively |
| First request | Slow (miss → origin fetch) | Fast (pre-cached) |
| Best for | Most websites (90% of cases) | Netflix pre-positioning, game patches |
| Cost | Pay for misses | Pay for storage |
Cache Invalidation
| Strategy | Freshness | Complexity | Best For |
|---|---|---|---|
Versioned URLs (app.a1b2c3.js) | Instant | Low | JS, CSS, images |
TTL-based (max-age=3600) | Stale up to TTL | Low | General content |
| Purge API | Instant | Medium | Breaking news, corrections |
| Stale-while-revalidate | Slightly stale, always fast | Medium | High-traffic pages |
Interview Cheat Sheet
- "Static assets with content-hashed URLs + infinite TTL — URL IS the version, zero invalidation"
- "Dynamic content: short TTL (60s) + stale-while-revalidate + purge API for corrections"
- "CDN also terminates TLS, absorbs DDoS, and reduces origin bandwidth costs by 95%"
- "Pull model for everything unless we're pre-positioning large content (video releases)"
- "Multi-CDN (Cloudflare + Fastly) for resilience — DNS failover between them"
When to Use / When NOT to Use
| Use When | Don't Use When |
|---|---|
| Global user base | All users in one region near origin |
| Static/semi-static content | Highly personalized, per-user content |
| High traffic volume | Low traffic (< 1000 req/day) |
| Need DDoS protection | Internal-only services |