How do ETag and caching work?

Definition:

HTTP caching reuses a stored response when its cache rules allow it, reducing repeated work and data transfer. An ETag identifies a resource representation so the client can ask whether its saved version is still valid. Conditional requests allow the server to say “unchanged” without sending the full representation again.

Key points:

• An ETag identifies a version of a resource.

• The client sends If-None-Match; the server can return 304 if unchanged.

• Use cache rules carefully so private responses do not reach other users.

• Cache-Control: no-cache permits storage but requires validation; no-store asks caches not to store the response.

Interview point:

• Decide freshness and privacy separately; a fast cache that leaks another user's response is incorrect.