🌍 CDN Caching – Explained Practically
CDN caching is when static or semi-static content (like images, videos, JS/CSS, etc.) is stored on servers distributed across the globe, closer to users. This drastically reduces load times, bandwidth usage, and server strain.
🔧 How It Works
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User requests a resource (say, an image from your website).
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The request is routed to the nearest CDN edge server.
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If the edge server has the resource cached:
- It serves it directly – super fast.
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If not:
- It fetches it from the origin server, serves it to the user, and stores a copy for the next user.
💡 Example
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You host a blog at
yourblog.com. -
You use a CDN (like Cloudflare, Akamai, or AWS CloudFront).
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A user in Germany requests an image → Served from a Frankfurt edge node.
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A user in India → Served from a Mumbai node.
Same image, but different cache locations closer to the user = 🔥 faster experience.
🧠 What Gets Cached in a CDN
| Type of Content | Cached? |
|---|---|
| HTML documents | Sometimes (depends on headers) |
| CSS / JS | Yes |
| Images / Fonts | Yes |
| Videos / PDFs | Yes |
| API responses | Can be (with proper headers) |
🛠️ Controlled via HTTP Headers
CDNs obey (or override) headers like:
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Cache-Control-
public, max-age=86400→ cache for 1 day -
no-cache→ check with origin every time
-
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ETag/Last-Modified- Helps with conditional requests
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Vary- Allows separate cache versions per user-agent, language, etc.
⏱️ Cache Invalidation
What if your content changes?
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Time-based: Use short TTLs (
max-age=60). -
Manual purge: Invalidate files via CDN dashboard/API.
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Versioning: Add a hash to filenames (
style.abc123.css) – new name = new cache.
📈 Benefits
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⚡ Speed: Reduced latency for global users.
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📉 Load: Offloads traffic from your origin server.
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💰 Cost: Lower bandwidth and compute usage.
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🔐 Security: Some CDNs protect against DDoS, add HTTPS, etc.
⚠️ Limitations / Gotchas
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Dynamic content caching needs careful setup.
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Stale cache issues if you don’t version or purge well.
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CDNs usually don’t cache POST responses (only GET).
🧪 Real-World Use Case
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Hosting product images on a fashion app.
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CDN caches them globally.
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When new images are uploaded → you purge the old ones via API or add versioning to filenames.