Value of "Expires" header differ for caching and content serving servers

[Maxim Dounin]

Hello!

On Thu, Apr 07, 2011 at 02:54:32PM -0500, Ryan Malayter wrote:

> On Thu, Apr 7, 2011 at 7:15 AM, Maxim Dounin <mdounin at mdounin.ru> wrote:
> > Hello!
> >
> > On Wed, Apr 06, 2011 at 06:47:10AM -0400, i.s.ivanov2 wrote:
> >
> >> I have a following servers structure:
> >> content.server contains an Apache instance, which processes one simple
> >> PHP script. On the same node Nginx is set as a caching proxy to that
> >> Apache.
> >> caching.server contains Nginx with proxy_cache directive, with upstream
> >> set to content.server
> >>
> >> Php script generates an XML file, that has a lifetime of 30 minutes.
> >> This is achieved by setting headers in PHP code: "Cache-Control:
> >> max-age=1800", "Date: now()" and "Expires: now()+1800seconds".
> >>
> >> content.server serves the cached file correctly:
> >> >   Date      Wed, 06 Apr 2011 09:50:49 GMT
> >> >   Cache-Control     public, must-revalidate, max-age=1800
> >> >   Expires   Wed, 06 Apr 2011 10:14:25 GMT
> >> >   Etag      26ee7051c360ee28d2a5697fb2b97a03
> >
> > This doesn't actually correct: Cache-Control max-age and Expires disagree.
> > According to max-age, document should expire at 10:20:49, while
> > expires set to 10:14:25.
> >
> > Unless configured to ignore some headers (with
> > proxy_ignore_headers directive) nginx will use first relevant
> > header it sees to actually expire cached document, Cache-Control
> > max-age in this case.
> 
> Really? That behavior would seem to be at odds with the HTTP 1.1
> specification. From RFC 2616 section 14.9.3:
> 
> "If a response includes both an Expires header and a max-age
> directive, the max-age directive overrides the Expires header, even if
> the Expires header is more restrictive"

Yes, I'm aware of the fact that it's not exactly correct per 
RFC2616.  The excuse is that nginx is HTTP/1.0 client.  ;)

> As almost all origin servers send both Expires and Cache-Control in
> their default configurations, with the Expires header coming first,
> wouldn't nginx end up doing the wrong thing most of the time?

I believe most servers either don't sent Expires/Cache-Control at 
all, or they agree.

If you do really care - workaround is to use proxy_ignore_headers.

Maxim Dounin