How CDNs Work

Also Known As

TL;DR

Explanation

Anycast routing: multiple edge servers share the same IP address; BGP routes requests to the geographically closest PoP. Cache miss: the edge fetches from origin and caches the response. Cache key: URL + Vary headers. Benefits: latency reduction, throughput offload from origin, reliability when origin is down, DDoS absorption. PHP sends Cache-Control: public, s-maxage=N to enable CDN caching. Dynamic API responses with correct headers are CDN-cacheable too.

Common Misconception

Why It Matters

Common Mistakes

• No Cache-Control headers — CDN cannot cache
• Caching Set-Cookie responses — serves another user's cookie
• Vary: * disables all caching
• Same URL different content without Vary header

Code Examples

// No cache headers — CDN pass-through:
public function products(): JsonResponse {
    return response()->json(Product::all());
    // Every request hits PHP and database — CDN provides no benefit
}

// CDN-cacheable response:
public function products(): JsonResponse {
    $etag = md5($this->productService->getLastModified());
    return response()
        ->json($this->productService->all())
        ->withHeaders([
            'Cache-Control' => 'public, s-maxage=300, stale-while-revalidate=60',
            'ETag'          => $etag,
            'Vary'          => 'Accept-Encoding',
        ]);
    // CDN serves for 5 minutes without touching PHP
}

References

• ↗ https://www.cloudflare.com/learning/cdn/what-is-a-cdn/

Tags

Related Terms