A short primer on HTTP caching headers

HTTP caching is governed primarily by the Cache-Control response header (RFC 9111). The most commonly seen directives are:

Freshness

• max-age=N — response is fresh for N seconds.
• s-maxage=N — same, but only for shared (CDN/proxy) caches.
• no-cache — cache may be stored, but must revalidate before reuse.
• no-store — do not cache at all.

Revalidation

When a stored response becomes stale, the cache sends a conditional request using If-None-Match (with an ETag) or If-Modified-Since.

Further reading

See RFC 9111 for the normative spec.