Logging a variable set by nginx's Lua module

Question

I am trying to use the Lua module in nginx to set a variable ("foo") based on JSON in the body of a request. Then I want to log the value of that variable to the access log.

Like so:

http {
    log_format mylogfmt '$remote_addr - $remote_user [$time_local] \
        "$request" $status $body_bytes_sent "$http_referer" \
        "$http_user_agent" "$foo"'
}

location / {
    proxy_pass http://remote-server.example.com/;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_connect_timeout 150;
    proxy_send_timeout 100;
    proxy_read_timeout 100;
    proxy_buffers 4 32k;
    client_max_body_size 8m;
    client_body_buffer_size 128k;

    rewrite_by_lua '
        cjson = require "cjson"
        ngx.req.read_body()
        body_table = cjson.decode(ngx.var.request_body)
        ngx.var.foo = body_table["foo"]
    ';

    access_log /var/log/nginx/access.log mylogfmt;
}

However, nginx won't start with this configuration. It complains thusly:

danslimmon@whatever:~$ sudo /etc/init.d/nginx reload
Reloading nginx configuration: nginx: [emerg] unknown "foo" variable
nginx: configuration file /etc/nginx/nginx.conf test failed

I tried adding a 'set $foo "-"' to the location, but that just seems to override what I'm doing in Lua.

Thoughts?

My nginx -V output

Answer 1

You need to define the variable $foo before the Lua module can use it. Check the doc for an example defining the variable within the location directive before utilizing it.