Cache-Control
The HTTP header that tells browsers and CDNs how long to cache a response and under what conditions to revalidate.
In long form.
Cache-Control directives include: max-age (seconds before stale), s-maxage (seconds for shared caches like CDNs), public / private (public allows shared caches, private = browser only), no-cache (must revalidate before serving), no-store (don't cache at all), stale-while-revalidate (serve stale while fetching fresh in background), immutable (asset never changes — common for hashed bundle filenames). Getting this right is often the highest-leverage performance tuning available.
Hashed static assets (JS bundles, CSS) get `Cache-Control: public, max-age=31536000, immutable` — they never change because the hash changes when the content does. HTML pages get `s-maxage=60, stale-while-revalidate=86400` — fresh from CDN within a minute, stale-served for a day if the origin is slow.
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