Is ETag returned in a 304 response?

Question

I'm building a node.js app which - reguarly, once a day - fetches data from some external web server (using "request" package).

I want to avoid fetching same data twice, so I keep track of each resource ETag when first downloading (200 status code).

Then, when fetching again (next day) that resource I add an If-None-Match header with the saved etag in the request.

Since I suspect to sometime receive a 200 status code (instead of expected 304) from the remote web server even if a resource contents is not modified, I ask if I should expect the resource ETag to be returned in a 304 response (and how to get it in the request response...), to try to debug this issue.

Physiography

Answer 1

Please have a look at appropriate RFC 2616.

The response MUST include the following header fields: (...)

• ETag and/or Content-Location, if the header would have been sent in a 200 response to the same request

So if ETag header is returned with 200 OK status code it also must be included in 304 Not Modified response.

Lory

What about the newer RFCs? Is it mentioned in them at all? – Fugere

Found it in tools.ietf.org/html/rfc7232#section-4.1 The server generating a 304 response MUST generate any of the following header fields that would have been sent in a 200 (OK) response to the same request: Cache-Control, Content-Location, Date, ETag, Expires, and Vary. – Fugere

Still true in the latest spec, rfc9110, section on 304. – Theola