System Design FAQ: Top Questions

A Content Delivery Network (CDN) caches and delivers static or dynamic content closer to users via a network of geographically distributed edge servers. CDNs help reduce latency, improve availability, and offload traffic from origin servers.

📋 Functional Requirements

• Serve static content (JS, CSS, images, videos)
• Geographically distributed caching
• Configurable cache expiration
• Origin fallback and purging

📦 Non-Functional Requirements

• Low latency and high throughput
• Scalability across regions
• DDoS mitigation and TLS support

🏗️ Architecture Components

• DNS-Based Routing: Edge selection using GeoDNS (e.g., Route 53, NS1)
• Edge Cache Nodes: NGINX, Varnish, or commercial CDNs like Cloudflare
• Origin Server: S3, GCS, or on-prem web servers
• Control Plane: Configuration management, invalidation APIs

🔁 Content Lifecycle

1. User requests https://cdn.example.com/logo.png
2. DNS resolves to nearest edge location
3. Edge checks cache → miss → fetch from origin
4. Cache response using TTL/header rules
5. Subsequent requests served from edge

🛠️ Real-World NGINX CDN Config Example

http {
  proxy_cache_path /data/nginx/cache levels=1:2 keys_zone=STATIC:10m inactive=24h max_size=1g;

  server {
    listen 80;
    server_name cdn.example.com;

    location / {
      proxy_pass http://origin.example.com;
      proxy_cache STATIC;
      proxy_cache_valid 200 302 10m;
      proxy_cache_use_stale error timeout invalid_header updating;
      add_header X-Cache-Status $upstream_cache_status;
    }
  }
}
        

📄 Cache Invalidation Patterns

• Time-based expiration (Cache-Control, Expires headers)
• Manual purge via API or CLI
• ETag/If-Modified-Since support

📈 Monitoring Metrics

• Cache hit/miss ratio
• Origin fetch latency
• Throughput per region
• HTTP status distribution (2xx/4xx/5xx)

🔐 Security Features

• TLS termination at edge
• DDoS protection and rate limiting
• WAF (Web Application Firewall)
• Token-based signed URLs (e.g., for private video)

🧰 Tools and Infrastructure

• Edge Stack: NGINX, Envoy, Varnish
• Cloud CDNs: Cloudflare, AWS CloudFront, GCP Cloud CDN
• Analytics: Datadog, Prometheus, Grafana

📌 Final Insight

Designing a CDN involves edge distribution, intelligent caching, and secure content delivery. Use cache headers wisely, monitor hit ratios, and protect origins with edge-first principles.