Recap: HTTP Caching and Conditional Requests

Cache-Control says who may keep a copy, and a validator lets client and server agree whether it is still current.

Caching headers decide where copies live; validators decide whether they are current.

Private caches serve one user, shared caches serve everyone; a fresh copy saves the trip.
max-age sets freshness; private keeps a response out of shared caches; no-store keeps it out of every cache.
no-cache stores and revalidates; it is not no-store.
ETag is a version label; If-None-Match uses weak comparison, If-Match strong.
A conditional GET that finds nothing changed gets 304 with no body.
Vary adds request headers to the cache key.
A lost update is prevented with If-Match, 412 on a mismatch and 428 when the header is missing.
Personal data in a shared cache is a breach; default to private.

Each header, and what it tells a cache or a server
Header	Sent by	Tells
Cache-Control	Server	Who may store the response and for how long
ETag	Server	The version of this representation
Last-Modified	Server	When it last changed
Vary	Server	Which request headers change the response
If-None-Match	Client	Send the body only if the version differs; 304 otherwise
If-Match	Client	Apply this write only if the version still matches; 412 otherwise

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