CDN Caching
When users are geographically distributed, latency becomes a major problem.
Even if your backend responds in milliseconds, network hops across continents can add hundreds of milliseconds.
Content Delivery Networks (CDNs) solve this by caching content at edge locations closer to users.
1. What is a CDN?
A CDN (Content Delivery Network) is a globally distributed network of edge servers that cache content.
Users are routed to the nearest edge node, reducing latency and offloading traffic from origin servers.
Typical flow
- User requests a resource (image, video, API).
- Request goes to nearest CDN edge node.
- If cache hit → return cached content.
- If cache miss → fetch from origin → cache at edge → return to user.
2. What Can CDNs Cache?
- Static Content
- HTML, CSS, JS, images, videos, downloads.
- Dynamic Content (with rules)
- API responses, GraphQL queries, personalized pages (with cache keys, cookies, headers).
- Edge Compute
- Some CDNs (e.g., Cloudflare Workers, AWS Lambda@Edge) can run code at edge.
3. CDN Caching Strategies
3.1 Cache-Control Headers
Cache-Control: max-age=3600→ cache for 1 hour.Cache-Control: no-cache→ validate before serving.Cache-Control: no-store→ don’t cache.
3.2 Content Invalidation
- Purge cache manually (e.g., after product update).
- Invalidate based on URL patterns, keys, or tags.
3.3 Cache Keys
- Define what makes a response unique (e.g.,
/product?id=123&lang=en). - Can include query params, cookies, or headers.
3.4 Stale-While-Revalidate
- Serve stale content immediately.
- In background, fetch fresh content and update cache.
4. Benefits of CDN Caching
- Lower latency → serve from nearby edge.
- Reduced load on origin servers.
- Higher availability (CDNs often absorb DDoS).
- Scalability: handle sudden traffic spikes (product launches, viral videos).
- Cost savings: fewer requests hitting origin.
5. Limitations & Pitfalls
- Personalized content is harder to cache.
- Stale content may be served if invalidation fails.
- Cache stampede: many requests miss cache at same time → origin overload.
- Cost: CDNs charge for data transfer.
6. Example Use Cases
- Video streaming (Netflix, YouTube).
- Static websites (CDNs like Vercel, Netlify).
- E-commerce product images.
- News sites with traffic spikes.
- Global SaaS applications serving APIs worldwide.
7. Interview Tips
- Always mention CDNs when talking about global scalability.
- State that CDNs reduce latency + origin load.
- Be aware of cache invalidation strategies.
- Mention edge computing for modern use cases.
8. Next Steps
- Learn about Cache Invalidation & Hot Keys.
- Explore Load Balancing in Networking.