Cloudflare's CDN caches automatically, but tuning for your specific use case dramatically improves performance. The Cache API gives you programmatic control—what to cache, for how long, and how to serve stale content.
Start with Cache-Control headers: s-maxage for CDN, max-age for browsers. Set vary headers correctly for content that differs by request characteristics, but minimize them—each variation multiplies your cache storage. Workers can also generate custom cache keys.
Stale-while-revalidate and stale-if-error are your friends: serve cached content while fetching freshness in the background or when your origin is down. Edge-specific TTL (s-maxage) overrides origin headers. Purge by URL, tag, or host.