Caching Strategies: Varnish, Redis, and CDN

Caching Layers

A complete caching strategy uses multiple layers, each optimized for different content types:

1. Browser cache — static assets (CSS, JS, images)
2. CDN — global edge caching
3. Reverse proxy cache — Varnish or Nginx FastCGI cache
4. Application cache — Redis or Memcached
5. Database cache — query cache, buffer pool

Varnish (HTTP Accelerator)

Varnish sits in front of your web server and caches full HTTP responses:

sudo apt install -y varnish

Edit /etc/varnish/default.vcl:

backend default {
    .host = "127.0.0.1";
    .port = "8080";
}

sub vcl_recv {
    # Don't cache POST requests
    if (req.method == "POST") { return (pass); }

    # Don't cache authenticated users
    if (req.http.Cookie ~ "session") { return (pass); }
}

sub vcl_backend_response {
    set beresp.ttl = 1h;
}

Varnish can serve cached pages in microseconds — 100-1000x faster than generating dynamically.

Redis (Application Cache)

# Cache database queries
$cacheKey = "user:profile:" . $userId;
$cached = $redis->get($cacheKey);
if ($cached) return json_decode($cached);

$user = $db->fetch("SELECT * FROM users WHERE id = ?", [$userId]);
$redis->setex($cacheKey, 300, json_encode($user));
return $user;

CDN (Content Delivery Network)

A CDN caches your content on servers worldwide, reducing latency for global visitors. Popular options:

• Cloudflare — free tier, DNS-based setup
• BunnyCDN — affordable, fast
• AWS CloudFront — integration with AWS

Cache Invalidation

The hardest part of caching. Common strategies:

• TTL-based — cache expires after a set time
• Event-based — purge cache when data changes
• Versioned URLs — style.css?v=13