Lesson 21 / 24

Caching with ETags

Q: A client sends `If-None-Match` with an ETag that still matches the current resource. What should the server return?

304 Not Modified with no body. A matching ETag means the client's cached copy is still valid, so the server confirms freshness with 304 and skips resending the body.

Letting clients skip re-downloading data that hasn't changed.

Cache-Control sets the rules

Cache-Control: max-age=60 tells clients and proxies a response is fresh for 60 seconds — no request needed at all in that window. no-store says never cache it (sensitive data).

ETags for revalidation

An ETag is a fingerprint of the current representation. The client resends it as If-None-Match; if unchanged the server replies 304 Not Modified with no body — saving bandwidth while confirming freshness.

The revalidation round-trip

The client always sends the request, but a match keeps the body off the wire.

GET /orders/482 HTTP/1.1
If-None-Match: "a1b2c3"

HTTP/1.1 304 Not Modified
ETag: "a1b2c3"

Quick check: A client sends `If-None-Match` with an ETag that still matches the current resource. What should the server return?

200 OK with the full body
304 Not Modified with no body
404 Not Found

Answer

304 Not Modified with no body — A matching ETag means the client's cached copy is still valid, so the server confirms freshness with 304 and skips resending the body.