Using memcache to set variables

[Valery Kholodkov]

----- Marcus Clyne <maccaday at gmail.com> wrote:
> Hi,
> 
> Avleen Vig wrote:
> > On Aug 12, 2009, at 6:08, Johan Bergström <johan at bergstroem.nu> wrote:
> >
> >> Hey,
> >> On Aug 8, 2009, at 05:13 , Avleen Vig wrote:
> >>
> >>> I started using the geo module to direct users by IP address, this
> >>> week, and really like it!
> >>>
> >>> It got me thinking about another thing I've been wanting to do: Direct
> >>> users to different machines based on some arbitrary data. I don't want
> >>> to rely just on cookies, because those can be manipulated. Here's what
> >>> I was thinking, and I'm wondering if it can be done some how in nginx?
> >>>
> >>> The sessionid is stored in a cookie. This can't be manipulated or the
> >>> users ends up not being logged in.
> >>> Store the sessionid in memcache, and have the value set to the name of
> >>> a location.
> >>> Have nginx fetch the value for 'sessionid' from memcache, and then
> >>> redirect the user internally to that location.
> >>>
> >>> Is this even remotely possible? I think some hardware load balancers
> >>> can do it, but I'm too poor to buy one of those :-)
> >>
> >> We're doing something similar - through a FastCGI-app written in C - 
> >> which looks up a host as key in memcached (cookie would work just as 
> >> fine) and returns a backend (we have lots of different nginx 
> >> backends) that the frontends proxies to. The key here is to use 
> >> X-Accel-Redirect. So, in short: Yes, it's possible and delivers ~2k 
> >> r/s per frontend for us. A FastCGI/whatever app written in php/python 
> >> would probably work just as well, port to C when performance is needed.
> >
> > Hi Johan!
> >
> > Thus is exactly what I'm hoping to do!
> > How do you use x-accel-redirect to get the name of a backend? I've 
> > never used it. Do ou have any code examples? This would be really 
> > perfect.
> Have a look at the eval module written by Valery Kholodkov - as mentioned.
> 
> A possible setup might be:
> 
> eval $location {
>     set $memcached_key   $cookie_sid;
>     memcache_pass   name:11211;
> }
> 
> location / {
>     try_files   $location @fallback;
> }
> 
> 
> location /_/ {
>     # add directives here if desired
>     internal;
> }
> 
> 
> @fallback {
>     [directives to generate session ID or location]
>     fastcgi_pass ...
> }
> 
> 
> I've not tested this config, though, so I don't guarantee it will work, 
> but something like it should work I think.  I added the /_/ location to 
> avoid infinite loops in the location / {} block, though there are many 
> ways you could achieve the same result ( locations returned from 
> memcached would have /_/-prefixed URLs).

For this task X-Accel-Redirect is more suitable.

> I think this should be quicker than passing off to fastcgi then 
> accessing memcached, since there would only be one of those two 
> overheads (and the overhead of sending a subrequest to memcached would 
> be very slight).
> 
> Hope this is useful,

-- 
Regards,
Valery Kholodkov