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CDN vs Direct Server Serving - System Design Interview Guide

Table of Contents

?? Core Idea (1-line difference)

  • CDN ? Serve from nearest edge (global, cached)
  • Direct Server ? Serve from origin (single location)

?? Script

�CDN brings data closer to users, while direct serving delivers everything from a single origin.�

?? CDN (High Signal)

? What it does

  • Distributed edge servers globally

  • Caches:

    • Images, videos, JS, CSS
    • Sometimes APIs (via edge caching)

  • Routes user to nearest location

?? Key Insight

?? Optimizes latency + scalability + bandwidth

??? Architecture Signals (Use CDN when)

  • Global users ??
  • High traffic / spikes
  • Static content heavy (images, videos)
  • Need fast load times (<100ms)

? Problems

  • Cache invalidation complexity ?
  • Extra cost
  • Not ideal for highly dynamic data

?? FAANG Q&A

Q1: Why CDN improves speed? ?? Data served from nearest edge ? lower latency.

Q2: What is cache hit ratio? ?? % of requests served from CDN cache.

Q3: How CDN reduces backend load? ?? Offloads traffic from origin server.

?? Script

�I use a CDN to cache and serve content from edge locations, reducing latency and offloading traffic from the origin server.�

?? 3. Direct Server Serving (High Signal)

? What it does

  • All requests go to single origin server
  • No intermediate caching layer

?? Key Insight

?? Simple but limited scalability

??? Architecture Signals (Use when)

  • Small/local application
  • Low traffic
  • Mostly dynamic data
  • Budget constraints

? Problems

  • High latency for distant users
  • Server overload risk
  • Poor scaling

?? FAANG Q&A

Q1: Why slow globally? ?? Users far from server ? high RTT.

Q2: Scaling issue? ?? All traffic hits one server.

?? Script

�Direct server serving is simple but doesn�t scale well for global users or high traffic.�

?? Key Differences (Interview Table)

Aspect CDN Direct Server
Location Global edges Single origin
Latency Low High (for distant users)
Scalability High Limited
Cost Higher Lower
Use Case Global apps Small/local apps

?? Script

�CDN improves performance and scalability globally, while direct serving is simpler but limited.�

?? 5. Real Architecture (FAANG Level)

? Standard Production Setup

Client
  ?
CDN (Edge Cache)
  ?
Origin Server (Backend)
  ?
Database

?? Why?

  • CDN ? fast delivery
  • Origin ? source of truth

?? FAANG Q&A

Q: What happens on cache miss? ?? CDN fetches from origin ? caches ? serves.

Q: How to update content? ?? Cache invalidation / TTL expiry.

?? Script

�In production, CDN sits in front of the origin server, serving cached content and reducing backend load.�

?? 6. Strong Signals vs Weak Signals

?? Choose CDN (Strong Signals)

  • �Global audience�
  • �High traffic�
  • �Media-heavy app�
  • �Low latency required�

?? Choose Direct Server (Strong Signals)

  • �Small app�
  • �Local users�
  • �Low traffic�
  • �Mostly dynamic content�

?? Weak Signals

  • �Startup ? no CDN� ?
  • �Dynamic app ? no CDN� ? (CDN still helps static parts)

?? 7. FAANG-Level Insights (Added)

?? Cache-Control Headers

  • max-age, public, private

?? 2. CDN Types

  • Push CDN (upload content)
  • Pull CDN (fetch on demand)

?? 3. Edge Computing

  • Run logic at CDN edge (Cloudflare Workers)

?? 4. Dynamic Content Acceleration

  • CDN optimizes even non-cacheable requests

?? Final Ultra-Short Summary

?? Golden Line

�CDN brings content closer to users; direct serving makes users come to the content.�