Honoring ETag of cached content

[Maxim Dounin]

Hello!

On Sat, Nov 02, 2013 at 02:55:09AM +0100, Marc Aymerich wrote:

> On Sat, Nov 2, 2013 at 12:16 AM, Maxim Dounin <mdounin at mdounin.ru> wrote:
> 
> > Hello!
> >
> > On Fri, Nov 01, 2013 at 10:29:06PM +0100, Marc Aymerich wrote:
> >
> > > Hi,
> > > I'm using nginx proxy pass to cache content of our dynamic web
> > application.
> > >
> > > In order to save some bandwidth our client application uses conditional
> > > requests based on ETag. However nginx ignores the ETag of cached pages :(
> > >
> > > What would be required if I want to honor the ETag of cached pages? Is
> > > something that can be achieve by means of configuration? or I would need
> > to
> > > write an nginx module in C?
> >
> > It is expected to just work in 1.3.3+.  If you don't see it
> > working, you may want to check the version you are running.
> 
> 
> Hi maxim,
> yeap I'm running 1.4.1 but I never get a 304 :(
> This is what a response from cache looks like
> 
> # wget --no-check https://[fdf5:5351:1dfd:0:0:0:0:2]/api/ -S --header
> 'If-None-Match: "83393a3900e4abce27212d7a27cae589"' -q
>   HTTP/1.1 200 OK
>   Server: nginx/1.4.1
>   Date: Sat, 02 Nov 2013 01:40:24 GMT
>   Content-Type: application/json
>   Transfer-Encoding: chunked
>   Connection: keep-alive
>   ETag: "83393a3900e4abce27212d7a27cae589"
>   Allow: GET, HEAD, OPTIONS
>   Vary: Accept, Cookie, Accept-Encoding
>   Expires: Sat, 02 Nov 2013 01:41:24 GMT
>   Cache-Control: max-age=60

[...]

> Notice the etag value from cached content is different from the fresh one,
> even though the content is exactly the same, I presume this is nginx doing
> its job of updating the ETag for some differences because of caching, but
> still fails to reply a conditional request properly :(

Ok, I see what goes on here.  Condintional requests (even ones 
using ETag in If-None-Match) are only handled by nginx if there is 
Last-Modified response date set, and it's not set in your backend 
responses.

This needs to be addressed, If-None-Match / If-Match handling from 
cache shouldn't require Last-Modified being present in a cached 
response.  Meanwhile, you may add some Last-Modified to your 
backend responses as a workaround.

-- 
Maxim Dounin
http://nginx.org/en/donation.html