proxy_set_header variable evaluation

[Maxim Dounin]

Hello!

On Thu, Aug 27, 2009 at 12:04:33AM +0530, Arvind Jayaprakash wrote:

> I am trying to do some tricks with upstream+proxy and ran into what
> seems like a limitation of the proxy_set_header feature.
> 
> When an upstream's response triggers resending the request to the next
> upsteam, I was hoping $upstream_response_time is available with data of
> what happened in the previous upsteams. I'm trying to pass it using the
> following directive:
> 
> proxy_set_header X-retry1 $upstream_response_time;
> 
> It looks like the variable is always unavailable (even when when nginx
> tries to access the second upstream for the same request). However, the
> value seems to be available during the logging phase.

This won't work because request to backend server created only 
once - and then nginx just tries to send the same request to 
available upstreams.

You may want to use error_page fallback instead of 
proxy_next_upstream, this will allow you to recreate request with 
new headers.  Note that you have to save $upstream_request_time to 
some temporary variable before upstream module will start again - 
as it will reset all related data.  E.g.

    upsteam backend {
        server 192.168.1.1:80;
        server 192.168.1.2:80;
        ...
    }

    server {
        ...

        proxy_next_upstream off;

        location / {
            error_page 502 504 = @fallback;
            proxy_pass http://backend;
        }

        location @fallback {
            set $blah $upstream_response_time;
            proxy_pass http://backend;
            proxy_set_header X-Blah $blah;
        }
    }

Note that you may use arbitrary fallback chain this way (including 
usage of spare backends and so on).

Maxim Dounin

> 
> I tried recompiling with the response variable marked as uncachable (v->no_cacheable = 1) but that did not seem to help.
> 
> The question is if this is a limitation of proxy_set_header or upstream
> variables? I got a feeling that $upstream_response_time is evaluated
> once before the upstreams are tried and evalutes to "not found" and not
> re-evaluated for each upstream attempt.
>