Re: --etag-compare vs. --time-cond

On 04-Mar-23 15:12, Paul Gilmartin via curl-users wrote:
> If I specify both --etag-compare and --time-cond and they disagree on
> whether a file is up-to-date, which wins?
>
> (It might depend on the HTTPD)

RFC 7232 <https://www.rfc-editor.org/rfc/rfc7232> Section3.3 specifies 
that the E-Tag wins for GET class requests.

> 3.3 <https://www.rfc-editor.org/rfc/rfc7232#section-3.3>. 
> If-Modified-Since
>
>     The "If-Modified-Since" header field makes a GET or HEAD request
>     method conditional on the selected representation's modification date
>     being more recent than the date provided in the field-value.
>     Transfer of the selected representation's data is avoided if that
>     data has not changed.
>
>       If-Modified-Since = HTTP-date
>
>     An example of the field is:
>
>       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT
>
>     A recipient MUST ignore If-Modified-Since if the request contains an
>     If-None-Match header field; the condition in If-None-Match is
>     considered to be a more accurate replacement for the condition in
>     If-Modified-Since, and the two are only combined for the sake of
>     interoperating with older intermediaries that might not implement
>     If-None-Match.
It's slightly more complicated in that there are rules about weak and 
strong validators, and whether Last-Modified is weak or strong is a bit 
subtle.  You have to read the RFC to really understand how this works.

But the simple answer is E-Tag wins.  Both are provided for 
compatibility with older servers and older clients.  Often LM is used 
because generating a strong E-Tag can be expensive.

3.4 says the same for update class requests. such as POST or DELETE.

Again, there are subtleties worth understanding.


Timothe Litt
ACM Distinguished Engineer
--------------------------
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.

-- 
Unsubscribe: https://lists.haxx.se/mailman/listinfo/curl-users
Etiquette:   https://curl.se/mail/etiquette.html