CDN (Content Delivery Network): Comprehensive Guide to Global Content Distribution

Introduction

A Content Delivery Network (CDN) is a distributed network of servers that delivers web content to users based on their geographic location. CDNs improve performance, reduce latency, and increase availability by caching content at edge locations closer to end users.

This guide covers:

CDN Fundamentals: Core concepts, architecture, and how CDNs work
Caching Strategies: Cache control, invalidation, and optimization
Edge Computing: Processing at the edge for lower latency
Use Cases: Static assets, dynamic content, media streaming
Best Practices: Performance, security, and cost optimization

What is a CDN?

A Content Delivery Network (CDN) is a geographically distributed network of servers that:

Caches Content: Stores copies of content at edge locations
Reduces Latency: Serves content from locations closer to users
Improves Performance: Faster content delivery and page load times
Increases Availability: Redundancy and failover capabilities
Reduces Origin Load: Offloads traffic from origin servers

Key Concepts

Origin Server: The original server that hosts the content

Edge Server: CDN server located close to end users

Cache Hit: Content served from CDN cache (fast)

Cache Miss: Content fetched from origin server (slower)

TTL (Time To Live): How long content is cached

Cache Invalidation: Removing content from cache before TTL expires

Edge Location: Geographic location of CDN servers (PoP - Point of Presence)

Origin Shield: Additional caching layer between edge and origin

Architecture

High-Level Architecture

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│   User      │────▶│   User      │────▶│   User      │
│  (US East)  │     │  (Europe)   │     │  (Asia)     │
└──────┬──────┘     └──────┬──────┘     └──────┬──────┘
       │                    │                    │
       └────────────────────┴────────────────────┘
                            │
                            │ HTTP/HTTPS Request
                            │
       ┌────────────────────┴────────────────────┐
       │                                         │
┌──────▼──────┐                         ┌───────▼──────┐
│  Edge       │                         │  Edge        │
│  Server     │                         │  Server      │
│  (US)       │                         │  (Europe)    │
│             │                         │              │
│  ┌────────┐ │                         │  ┌────────┐ │
│  │ Cache  │ │                         │  │ Cache  │ │
│  └────────┘ │                         │  └────────┘ │
└──────┬──────┘                         └───────┬──────┘
       │                                         │
       │ Cache Miss                              │ Cache Miss
       │                                         │
       └────────────────────┬────────────────────┘
                            │
                            ▼
              ┌─────────────────────────┐
              │   Origin Server         │
              │   (Content Source)      │
              └─────────────────────────┘

Explanation:

Users: End users in different geographic locations requesting content (e.g., web pages, images, videos).
Edge Servers: CDN servers located close to users that cache and serve content. Reduces latency by serving content from nearby locations.
Cache: Local storage on edge servers that stores frequently accessed content to avoid fetching from origin.
Origin Server: The original source of content (e.g., web server, application server) that edge servers fetch from when cache misses occur.

Core Architecture

┌─────────────────────────────────────────────────────────┐
│                    End Users                              │
│  (US, Europe, Asia, etc.)                                │
└────────────────────┬────────────────────────────────────┘
                     │
                     │ HTTP/HTTPS Requests
                     │
        ┌────────────┴────────────┐
        │                         │
┌───────▼────────┐      ┌─────────▼──────────┐
│  Edge Server   │      │   Edge Server      │
│  (US East)     │      │   (Europe)         │
│                │      │                    │
│  ┌──────────┐  │      │  ┌──────────┐     │
│  │  Cache   │  │      │  │  Cache   │     │
│  └──────────┘  │      │  └──────────┘     │
└───────┬────────┘      └─────────┬──────────┘
        │                         │
        │ Cache Miss              │ Cache Miss
        │                         │
        └────────────┬────────────┘
                     │
                     │ Origin Requests
                     │
        ┌────────────▼────────────┐
        │    Origin Shield         │
        │    (Regional Cache)      │
        └────────────┬─────────────┘
                     │
                     │ Cache Miss
                     │
        ┌────────────▼────────────┐
        │    Origin Server         │
        │    (Application Server)  │
        └─────────────────────────┘

How CDNs Work

1. Content Request Flow

Step 1: User Request

User requests content (e.g., https://example.com/image.jpg)
DNS resolves to nearest edge server

Step 2: Edge Server Check

Edge server checks cache
Cache Hit: Serves from cache (fast)
Cache Miss: Fetches from origin

Step 3: Origin Fetch (if cache miss)

Edge server requests from origin
Origin server responds with content
Edge server caches content
Edge server serves to user

Step 4: Subsequent Requests

Same content served from cache
Much faster response time

2. DNS-Based Routing

CDNs use DNS to route users to nearest edge server:

User Request → DNS Query → CDN DNS Server
                              ↓
                    Geographic Routing
                              ↓
                    Nearest Edge Server
                              ↓
                    IP Address Returned
                              ↓
                    User Connects to Edge

Geographic Routing Methods:

GeoIP: Based on user’s IP location
Anycast: Same IP address, routes to nearest server
DNS-based: DNS server returns nearest edge IP

3. Caching Strategy

Cacheable Content:

Static assets (images, CSS, JS)
Media files (videos, audio)
API responses (with appropriate headers)
HTML pages (with cache control)

Cache Headers:

Cache-Control: public, max-age=3600
ETag: "abc123"
Last-Modified: Wed, 21 Oct 2024 07:28:00 GMT

Cache Control Directives:

public: Can be cached by any cache
private: Only browser can cache
no-cache: Must revalidate before serving
max-age: Time in seconds content is fresh
s-maxage: Max age for shared caches (CDN)

CDN Features

1. Static Content Delivery

Use Cases:

Images, CSS, JavaScript files
Fonts, icons, logos
Static HTML pages

Benefits:

Fast delivery from edge
Reduced origin load
Global distribution

Example:

Origin: https://example.com/static/image.jpg
CDN: https://cdn.example.com/image.jpg

2. Dynamic Content Acceleration

Use Cases:

API responses
Personalized content
Real-time data

Techniques:

Edge Caching: Cache dynamic content with short TTL
ESI (Edge Side Includes): Combine cached and dynamic content
Query String Handling: Cache based on query parameters

Example:

GET /api/user/123?cache=true
Cache-Control: public, max-age=60

3. Media Streaming

Use Cases:

Video on demand (VOD)
Live streaming
Audio streaming

Features:

Adaptive Bitrate: Adjust quality based on bandwidth
Chunked Delivery: Stream in segments
Multi-Format Support: HLS, DASH, etc.

Example:

Video: https://cdn.example.com/video/playlist.m3u8
Segments: segment1.ts, segment2.ts, ...

4. Security Features

DDoS Protection:

Absorbs attack traffic at edge
Protects origin servers
Rate limiting and filtering

SSL/TLS Termination:

HTTPS at edge
Certificate management
TLS version control

WAF (Web Application Firewall):

SQL injection protection
XSS protection
Bot detection

Access Control:

Signed URLs
Token authentication
Geographic restrictions

5. Edge Computing

Use Cases:

Serverless functions at edge
Request/response modification
A/B testing
Personalization

Benefits:

Lower latency
Reduced origin load
Global processing

Example:

Edge Function:
- Modify headers
- Add authentication
- Transform responses
- Route requests

CDN Providers

Major CDN Providers

Cloudflare:

Global network (200+ cities)
Free tier available
DDoS protection
Edge computing (Workers)

Amazon CloudFront:

AWS integration
Global edge locations
Lambda@Edge
Real-time metrics

Fastly:

Real-time purging
Edge computing (VCL)
High performance
Developer-friendly

Akamai:

Largest network
Enterprise focus
Advanced security
Media delivery

Google Cloud CDN:

GCP integration
Global network
Load balancing integration
Cost-effective

Cache Invalidation

Methods

1. TTL-Based Expiration

Cache-Control: max-age=3600

Content expires after TTL
Automatic refresh
Simple to implement

2. Manual Purging

PURGE /path/to/content

Immediate invalidation
API-based
Granular control

3. Versioning

/image.jpg?v=123
/style.css?v=456

URL-based versioning
No invalidation needed
Cache busting

4. Tag-Based Invalidation

Cache-Tag: product-123, category-456

Invalidate by tags
Bulk operations
Efficient

Best Practices

Cache Invalidation Strategy:

Use versioning for static assets
Use TTL for dynamic content
Use purging for urgent updates
Monitor cache hit ratio

Example:

// Version static assets
const version = process.env.BUILD_VERSION;
const imageUrl = `https://cdn.example.com/image.jpg?v=${version}`;

// Set appropriate TTL
app.get('/api/data', (req, res) => {
  res.set('Cache-Control', 'public, max-age=300');
  res.json(data);
});

Performance Optimization

1. Cache Hit Ratio

Target: 90%+ cache hit ratio

Optimization:

Increase TTL for static content
Cache more content types
Use origin shield
Monitor and adjust

Metrics:

Cache Hit Ratio = (Cache Hits / Total Requests) × 100

2. Compression

Enable compression:

Gzip/Brotli compression
Reduce bandwidth
Faster delivery
Lower costs

Example:

Accept-Encoding: gzip, br
Content-Encoding: gzip

3. HTTP/2 and HTTP/3

Benefits:

Multiplexing
Header compression
Server push
Lower latency

4. Image Optimization

Techniques:

WebP format
Lazy loading
Responsive images
Compression

Example:

<picture>
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" alt="Image">
</picture>

Use Cases

1. Static Website Hosting

Benefits:

Fast global delivery
High availability
Cost-effective
Easy to scale

Example:

Static Site → CDN → Global Users

2. E-Commerce

Use Cases:

Product images
CSS/JavaScript
Fonts and icons
Static pages

Benefits:

Faster page loads
Better user experience
Higher conversion rates

3. Media Delivery

Use Cases:

Video streaming
Image galleries
Audio files
Downloads

Benefits:

Smooth streaming
Reduced buffering
Global reach

4. API Acceleration

Use Cases:

API responses
GraphQL queries
REST endpoints

Techniques:

Edge caching
Query string handling
Header-based caching

5. Mobile Applications

Use Cases:

App assets
Media content
API responses

Benefits:

Lower latency
Reduced data usage
Better performance

Best Practices

1. Cache Strategy

Static Content:

Cache-Control: public, max-age=31536000, immutable

Dynamic Content:

Cache-Control: public, max-age=300, must-revalidate

Private Content:

Cache-Control: private, no-cache

2. Security

HTTPS:

Always use HTTPS
HSTS headers
Certificate management

Access Control:

Signed URLs for private content
Token authentication
Geographic restrictions

WAF:

Enable WAF rules
Monitor threats
Update rules regularly

3. Monitoring

Key Metrics:

Cache hit ratio
Response time
Bandwidth usage
Error rates
Origin load

Tools:

CDN analytics
Real-time monitoring
Alerts and notifications

4. Cost Optimization

Strategies:

Optimize cache hit ratio
Use compression
Choose right pricing tier
Monitor usage
Use origin shield

5. Content Optimization

Techniques:

Minify CSS/JS
Optimize images
Use modern formats (WebP, AVIF)
Enable compression
Lazy load content

Code Examples

1. CDN Configuration (Nginx)

# Origin server configuration
server {
    listen 80;
    server_name example.com;
    
    location /static/ {
        # Set cache headers
        add_header Cache-Control "public, max-age=31536000, immutable";
        add_header Vary "Accept-Encoding";
        
        # Enable compression
        gzip on;
        gzip_types text/css application/javascript image/svg+xml;
        
        # Serve from CDN
        proxy_pass http://cdn.example.com;
    }
}

2. Cache Invalidation (API)

// Invalidate CDN cache
async function invalidateCDN(paths) {
  const response = await fetch('https://api.cdn.example.com/purge', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${API_KEY}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify({ paths })
  });
  
  return response.json();
}

// Usage
await invalidateCDN([
  '/static/image.jpg',
  '/api/products/123'
]);

3. Signed URLs

// Generate signed URL for private content
const crypto = require('crypto');

function generateSignedURL(path, expiresIn = 3600) {
  const expires = Math.floor(Date.now() / 1000) + expiresIn;
  const secret = process.env.CDN_SECRET;
  const stringToSign = `${path}${expires}`;
  const signature = crypto
    .createHmac('sha256', secret)
    .update(stringToSign)
    .digest('hex');
  
  return `https://cdn.example.com${path}?expires=${expires}&signature=${signature}`;
}

// Usage
const url = generateSignedURL('/private/video.mp4', 3600);

4. Cache Control Headers

// Express.js cache headers
app.get('/static/*', (req, res, next) => {
  // Static assets - long cache
  res.set('Cache-Control', 'public, max-age=31536000, immutable');
  res.set('ETag', generateETag(req.path));
  next();
});

app.get('/api/*', (req, res, next) => {
  // API responses - short cache
  res.set('Cache-Control', 'public, max-age=300, must-revalidate');
  res.set('Vary', 'Accept-Encoding');
  next();
});

app.get('/private/*', (req, res, next) => {
  // Private content - no cache
  res.set('Cache-Control', 'private, no-cache, must-revalidate');
  next();
});

What Interviewers Look For

CDN Understanding

CDN Fundamentals
- Understanding of how CDNs work
- Cache hit/miss concepts
- Geographic distribution
- Red Flags: No CDN understanding, wrong concepts, no geographic awareness
Caching Strategy
- Cache control headers
- TTL management
- Cache invalidation
- Red Flags: No caching strategy, wrong headers, no invalidation
Performance Optimization
- Cache hit ratio optimization
- Compression
- Content optimization
- Red Flags: No optimization, poor hit ratio, no compression

Problem-Solving Approach

Use Case Identification
- When to use CDN
- What content to cache
- Cache strategy selection
- Red Flags: Wrong use cases, caching everything, no strategy
Performance Analysis
- Cache hit ratio analysis
- Latency reduction
- Bandwidth optimization
- Red Flags: No metrics, no analysis, poor optimization

System Design Skills

Architecture Design
- CDN integration
- Origin server design
- Failover strategies
- Red Flags: No CDN, poor integration, no failover
Cost Optimization
- Bandwidth optimization
- Cache hit ratio
- Pricing understanding
- Red Flags: No cost awareness, poor optimization, high costs

Communication Skills

Clear Explanation
- Explains CDN concepts
- Discusses trade-offs
- Justifies design decisions
- Red Flags: Unclear explanations, no justification, confusing

Meta-Specific Focus

Performance Expertise
- Understanding of performance optimization
- CDN mastery
- Real-world application
- Key: Demonstrate performance optimization expertise
Global Systems
- Understanding of global distribution
- Latency optimization
- Geographic awareness
- Key: Show global system design skills

Summary

CDN Key Points:

Global Distribution: Content served from edge locations
Caching: Reduces latency and origin load
Performance: Faster content delivery and page loads
Scalability: Handles high traffic globally
Security: DDoS protection, WAF, SSL/TLS

Common Use Cases:

Static asset delivery (images, CSS, JS)
Media streaming (video, audio)
API acceleration
Global content distribution
Mobile app content

Best Practices:

Optimize cache hit ratio (90%+)
Use appropriate cache headers
Enable compression
Monitor performance metrics
Implement cache invalidation strategy
Use HTTPS and security features
Optimize content (images, code)

CDNs are essential for building high-performance, globally distributed applications with low latency and high availability.

Introduction

What is a CDN?

Key Concepts

Architecture

High-Level Architecture

Core Architecture

How CDNs Work

1. Content Request Flow

2. DNS-Based Routing

3. Caching Strategy

CDN Features

1. Static Content Delivery

2. Dynamic Content Acceleration

3. Media Streaming

4. Security Features

5. Edge Computing

CDN Providers

Major CDN Providers

Cache Invalidation

Methods

Best Practices

Performance Optimization

1. Cache Hit Ratio

2. Compression

3. HTTP/2 and HTTP/3

4. Image Optimization

Use Cases

1. Static Website Hosting

2. E-Commerce

3. Media Delivery

4. API Acceleration

5. Mobile Applications

Best Practices

1. Cache Strategy

2. Security

3. Monitoring

4. Cost Optimization

5. Content Optimization

Code Examples

1. CDN Configuration (Nginx)

2. Cache Invalidation (API)

3. Signed URLs

4. Cache Control Headers

What Interviewers Look For

CDN Understanding

Problem-Solving Approach

System Design Skills

Communication Skills

Meta-Specific Focus

Summary

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