proxy_set_header variable evaluation

[Arvind Jayaprakash]

On Aug 26, Maxim Dounin wrote:
>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).

This approach has the problem that all the upstream functionality like
max_tries, weight etc. etc. is lost.

Is the proxy module the right place to start hacking if I want to create
a new version of set_header where the variable gets evaluated for each
time a new upstream is selected for a new request?