System Design Learning Hub

Content Delivery Network (CDN)

How it works

CDN Overview
How it works
  1. DNS routes to edge
  2. Edge serves cached asset
  3. On miss, fetch from origin
  4. Cache with versioned keys

Overview

  • CDN caches content at edge locations to reduce latency and origin load.
  • Ideal for static assets, media, and cacheable API responses with proper headers.

When to use

  • Global audiences or mobile networks where RTT dominates.
  • Heavy static/media traffic; offload bandwidth from origin.

Trade-offs

  • Cache invalidation complexity; risk of stale content.
  • Additional cost and debugging complexity across edge layers.

Patterns

  • Versioned asset URLs (content hashing) for safe long TTLs.
  • Cache keys include path + query + device/lang; use Vary headers.
  • Stale-while-revalidate, prefetching, and signed URLs for protected content.
  • Tiered caching (edge ▶ regional ▶ origin) and origin shield.

Anti-patterns

  • Purging broad wildcards in production without safeguards.
  • No cache-busting/versioning causing clients to see stale assets.
  • Caching personalized/private responses without proper controls.

📐 Quick Diagram


      User ▶ DNS ▶ Edge POP ▶ (cache hit) ▶ Response
                        └▶ (miss) ▶ Shield ▶ Origin

❓ Interview Q&A (concise)

  • Q: Why are hashed filenames used? A: To enable immutable caching; a new hash = new URL, no purge needed.
  • Q: Dynamic APIs via CDN? A: Use short TTL or bypass; consider edge compute for auth and personalization.
  • Q: Reduce origin load? A: Origin shield, tiered cache, longer TTL with versioning, and compression.

🎯 What is a CDN?

A CDN is like having a network of local warehouses for a global business - it stores copies of your content closer to your users, reducing latency and load times. CDNs are crucial for delivering static content like images, videos, and scripts quickly and efficiently.
👤 User
⬇️
🌐 Nearest CDN Edge Server
⬇️
🏢 Origin Server

🚀 How CDNs Work

  1. User requests content
  2. DNS routes to nearest CDN edge server
  3. If content cached, serve immediately
  4. If not cached, fetch from origin server
  5. Cache content for future requests

CDN Benefits

  • Reduced Latency: Content served from nearby locations
  • Reduced Bandwidth: Less traffic to origin servers
  • Improved Availability: Multiple servers provide redundancy
  • DDoS Protection: Distributed infrastructure absorbs attacks

Types of Content

Static Content

  • Images, CSS, JavaScript files
  • Long cache times
  • Perfect for CDN

Dynamic Content

  • API responses, personalized content
  • Short cache times or no caching
  • Edge computing for processing

CDN Providers

  • Cloudflare: Global network, security features
  • AWS CloudFront: Integrated with AWS services
  • Akamai: Enterprise-focused, large network
  • Azure CDN: Microsoft's offering

Cache Control

  • TTL Settings: How long to cache content
  • Cache Headers: Control browser and CDN caching
  • Purging: Manually remove cached content
  • Invalidation: Mark content as stale