Condition pagespeed based on upstream response headers

[Ash Tyndall]

Hi there,

For my purposes, I only want to run pagespeed filters on "public" pages 

served through my application, as these are the pages I are concerned with 

the performance of / I want to avoid any potential issues with more dynamic

pages being rewritten.

Right now I are able to accomplish this with Apache and mod_pagespeed with 

the following configuration within pagespeed.conf;

    FilterDeclare pagespeed CONTENT_SET

    FilterProvider pagespeed MOD_PAGESPEED_OUTPUT_FILTER \

      "%{resp:X-Pagespeed-Enable} == 'true'"

    FilterChain pagespeed

This config only enables pagespeed  when the response from the 

application contains the header 'X-Pagespeed-Enable: true', which it emits 

when relevant.

However, I am struggling to do this with nginx. I have a test server setup 

with nginx proxying our application through a Unix socket with a configuration 

like this (simplified);

    upstream app {

      server unix:/foo.sock fail_timeout=0;

    }

    server {

      listen 80;

      server_name localhost example.com;

      root /app/public;

      try_files $uri/index.html $uri @app;

      # Pagespeed

      pagespeed on;

      pagespeed FileCachePath /var/ngx_pagespeed_cache;

      location @app {

        proxy_pass http://app;

        # Both X-Debug-1 and X-Debug-2 successfully contain the 

        # X-Pagespeed-Enable header returned by the server

        add_header "X-Debug-1" $upstream_http_x_pagespeed_enable;

        add_header "X-Debug-2" $sent_http_x_pagespeed_enable;

      }

      # Ensure requests for pagespeed optimized resources go to the pagespeed 

      # handler, and no extraneous headers get set.

      location ~ "\.pagespeed\.([a-z]\.)?[a-z]{2}\.[^.]{10}\.[^.]+" {

        add_header "" "";

      }

      location ~ "^/pagespeed_static/" { }

      location ~ "^/ngx_pagespeed_beacon$" { }

    }

However, I cannot figure out how to make pagespeed or its filters conditional 

on the value of the $upstream_http_x_pagespeed_enable /

$sent_http_x_pagespeed_enable (same value) variable.

I've tried using if statements similar to that outlined by the tests

in https://github.com/pagespeed/ngx_pagespeed/pull/641/files, as well as 

various permutations of if statements / maps / variables / etc, but I can't 

seem to find a working approach.

 

I suspect that the response variables aren't set at the point that they are 

being evaluated in the config, but I'm not well versed enough in nginx to say

for sure.

Does anyone have any suggestions / thoughts on how I might accomplish this?

Greatly appreciated,

Ash

[Otto van der Schaaf]

If you could add "PageSpeed:[on|off]" to the response headers with add_header in the @app location{} block, I think that should work.

See https://modpagespeed.com/doc/experiment#PerRequest 

Alternatively, ngx_pagespeed also allows you to script configuration variables, but PageSpeed:on|off is the one variable that is not yet support as that needs to be treated differently from the rest, but maybe a conditional RewriteLevel PassTrough mode may help you here.

https://modpagespeed.com/doc/system#nginx_script_variables

Otto

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[Ash Tyndall]

Thanks Otto, setting the "PageSpeed" header worked great!

For anyone curious, the following config maintains compatibility with my existing header (and to default pagespeed to off if the header is unset), and seems to otherwise work;

map $upstream_http_x_pagespeed_enable $pagespeed_header {

  "true"        "on";

  default       "off";

}

upstream app {

  server unix:/foo.sock fail_timeout=0;

}

server {

  listen 80;

  server_name localhost example.com;

  root /app/public;

  try_files $uri/index.html $uri @app;

  # Pagespeed

  pagespeed on;

  pagespeed FileCachePath /var/ngx_pagespeed_cache;

  location @app {

    proxy_pass http://app;

    

    # Conditionally enable pagespeed

    add_header "PageSpeed" $pagespeed_header;

  }

  # Ensure requests for pagespeed optimized resources go to the pagespeed 

  # handler, and no extraneous headers get set.

  location ~ "\.pagespeed\.([a-z]\.)?[a-z]{2}\.[^.]{10}\.[^.]+" {

    add_header "" "";

  }

  location ~ "^/pagespeed_static/" { }

  location ~ "^/ngx_pagespeed_beacon$" { }

}

Ash

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