How can I make nginx return a static response and send request headers to app?

Question

I am making a high-load web statistics system through embedding <img> tag to site. The thing I want to do is:

1. nginx gets request for an image from some host
2. it gives as answer to host little 1px static image from filesystem
3. at this time it somehow transfers request's headers to application and closes connection to host

I am working with Ruby and I'm going to make a pure-Rack app to get the headers and put them into a queue for further calculations.

The problem I can't solve is, how can I configure sphinx to give headers to the Rack app, and return a static image as the reply without waiting a for response from the Rack application?

Also, Rack is not required if there is more common Ruby-solution.

Answer 1

A simple option is to terminate the client connection ASAP while proceeding with the backend process.

server {
    location /test {
        # map 402 error to backend named location
        error_page 402 = @backend;

        # pass request to backend
        return 402;
    }

    location @backend {
        # close client connection after 1 second
        # Not bothering with sending gif
        send_timeout 1;

        # Pass the request to the backend.
        proxy_pass http://127.0.0.1:8080;
    }
}

The option above, while simple, may result in the client receiving an error message when the connection is dropped. The ngx.say directive will ensure that a "200 OK" header is sent and as it is an async call, will not hold things up. This needs the ngx_lua module.

server {
    location /test {
        content_by_lua '
            -- send a dot to the user and transfer request to backend
            -- ngx.say is an async call so processing continues after without waiting
            ngx.say(".")
            res = ngx.location.capture("/backend")

        ';
    }

    location /backend {
        # named locations not allowed for ngx.location.capture
        # needs "internal" if not to be public
        internal;

        # Pass the request to the backend.
        proxy_pass http://127.0.0.1:8080;
    }

}

A more succinct Lua based option:

server {
    location /test {
        rewrite_by_lua '
            -- send a dot to the user
            ngx.say(".")

            -- exit rewrite_by_lua and continue the normal event loop
            ngx.exit(ngx.OK)
        ';
        proxy_pass http://127.0.0.1:8080;
    }
}

Definitely an interesting challenge.