Caching and Conditional Requests (ETag)

Ask "has it changed?"

An ETag is an opaque version tag for a representation (often a hash). The server sends ETag: "abc"; later the client repeats the request with If-None-Match: "abc". If the resource has not changed, the server answers 304 Not Modified with no body, saving bandwidth and time. The same tags give optimistic concurrency for writes: the client sends If-Match: "abc" with a PUT/PATCH, and the server returns 412 Precondition Failed if someone else changed the resource first, preventing lost updates. Cache-Control (max-age, private, no-store) tells browsers and CDNs how long to reuse a response; mark responses containing personal data private or no-store.

A 304 from the real API, run

I ran this against a small real HTTP API built with only the Python standard library (full code in the case study). The first GET returns the item and an ETag. Repeating it with that tag as If-None-Match returns 304 with no body (None).

# uses srv, base and call() from the runnable demo in the case study
s, h, b = call("GET", "/v2/items/1"); tag = h["ETag"]; print(s, b, "has etag:", tag.startswith('"'))
s, h2, b2 = call("GET", "/v2/items/1", headers={"If-None-Match": tag}); print(s, b2)

Output:

200 {'id': 1, 'first_name': 'Asha', 'last_name': 'Rao', 'plan': 'pro'} has etag: True
304 None

Use If-Match to prevent lost updates

Two people edit the same record: without If-Match, the second save silently overwrites the first. With it, the second gets a clear 412 and can reload and merge.

Quick check: What does `304 Not Modified` tell the client?

• The cached copy is still valid; no body is sent
• The resource was deleted
• The request failed
• Credentials are wrong