How can I set variable within location?

[huwylphimet]

Hi,
How can I set variable within location (see @handle_redirects in the example below)?
In the template file I found the set key but in the comment it says Not available in the 'http' context

I would like to achieve something like that:

http {
    upstream api {
        server api1:8000;
        server api2:8000;
    }

    server {
        listen 80;

        location / {
            proxy_pass http://api;
            proxy_intercept_errors on;
            error_page 301 302 307 = @handle_redirects;
        }

        location @handle_redirects {
            set $original_location $upstream_http_location;  # how to configure the template.yml for setting variable here?
            proxy_pass $original_location;
        }
    }
}

Thanks for any advice

[huwylphimet]

Hello,
I found a solution myself by using the custom_directives key like the following:

          - location: /
            proxy:
              pass: "http://api"
              intercept_errors: true
            core:
              error_page:
                - code:
                    - 301
                    - 302
                    - 307
                  uri: "@handle_redirects"
          - location: "@handle_redirects"
            custom_directives:
              - set $original_location $upstream_http_location;
              - proxy_pass $original_location;

If someone would have a more elegant way, please share it. If not, at least maybe my solution could be useful to someone.

[alessfg]

Hey @huwylphimet!

You should be able to use set within the location block. When a comment says Not available in the 'http' context what it means is that it's not available directly within the http block, but it's available elsewhere (server and location in this case) 😄

[huwylphimet]

yes, you're right :) I forgot to defined the parent level rewrite...
now I've:

          - location: "@handle_redirects"
            rewrite:
              set:
                - variable: $original_location
                  value: $upstream_http_location
            proxy:
              pass: $original_location

that generates:

server {
    location @handle_redirects {
        proxy_pass $original_location;

        set $original_location $upstream_http_location;
    }
}

I just noticed that the directives are not in the same order:
I defined the set directive before the proxy_pass but the result is reversed.
Is it possible to set the order of directives in order to have the variable set before the proxy_pass directive? Or this doesn't matter for ningx at all in this case? (my goal would be to make ningx handle the http redirect 30x from the upstreams, see example in the 1st post)

[alessfg]

For most use cases the order of nginx directives does not matter. There are some edge cases (mostly involving using rewrite) where it will matter, but you should be good here. That being said, the $upstream_http_location isn't one of the default nginx variables as far as I know, so you might run into some issues if that variable is not set when nginx starts. (And if I might add a tiny further suggestion, if the above is not a simplified config you could just proxy_pass to $upstream_http_location directly.)

[huwylphimet]

Thank you very much for this feedback!
I'll check for the $upstream_http_location state in our production environment (until now I only tested that config in my local docker environment and I can confirm now that the order doesn't matter in this situation).
This config is likely to evolve in the future and storing $upstream_http_location might allow us to consume that information after proxy_pass is being called (since as soon as proxy_pass is called, the $upstream_http_location might be overwritten).
But for now, it is only used there and I will try, as you kindly suggested, to proxy_pass to $upstream_http_location directly and see if nginx is still doing what is expected.
Many thanks for your help.
Kind regards