• Why CDNs? Latency, offload, availability
• Topology: POPs, edges, mid-tier, origins
• Caching: keys, TTL, validation
• Invalidation, versioning, and cache busting
• Request routing: DNS, anycast, Geo/IP, ECMP
• Security: TLS, WAF, DDoS, bot mitigation
• HTTP/2, HTTP/3, compression, image optimization
• Operations and observability
• Exercises

• Reduce latency via proximity; offload origin bandwidth and compute.
• Improve resilience with regional isolation and multi-origin failover.
• Lower egress costs with edge caching and peering.

• Hierarchical caches: edge → regional/mid-tier → origin shield → origin.
• Anycast to nearest POP; DNS steering for geo/latency-aware routing.
• Active-active origins and shield layers reduce origin amplification.

• Cache key components: host, path, query, headers (Vary), device hints.
• Freshness: Cache-Control (max-age, s-maxage), Expires; revalidation via ETag/If-None-Match.
• Edge compute can rewrite keys and responses (workers/functions).

Cache-Control: public, max-age=600, s-maxage=3600

• Invalidate by URL/prefix, or version assets with content hashes for instant rollover.
• Stale-while-revalidate and stale-if-error improve resilience.
• Blue/green asset switch via manifest or immutable paths.

• Geo DNS maps client resolvers to nearby POPs; anycast BGP announces the same prefix globally.
• Latency-based routing and health checks steer traffic away from faults.
• Per-request redirects based on device/geo via edge logic.

• Edge TLS termination with modern ciphers; HSTS and OCSP stapling.
• WAF rules for OWASP Top 10; rate limiting; bot scoring and challenges.
• Automatic DDoS mitigation with scrubbing and anomaly detection.

• Enable Brotli/gzip; optimize images (WebP/AVIF) and responsive variants.
• Coalesce connections with H/2; migrate to H/3 for better loss resilience.
• Use signed exchanges, preconnect, and early hints where supported.

• Metrics: hit ratio, origin offload, TTFB by POP, error rates, invalidation latency.
• Logs/traces at edge and origin; correlate with synthetic monitoring.
• Plan capacity per POP; simulate regional failures; test cache warm strategies.

1. Design a CDN for a global video site: pick POP regions, caching rules, and routing.
2. Implement cache-busting with content hashes and verify fast rollbacks.
3. Measure hit ratio improvements after adding an origin shield layer.