How to add any variable to Kong access logs

How can I add extra details to the access log?

Using the following configurations, it is possible to add any value that can be set from a pre-function plugin, to the access logs.

In this example we will be adding the route name using the get_route() function from the Kong PDK.

This function returns a route object which allows access to the various properties of the route.

Specifically we will be using route.name to retrieve the route name.

1. Use a custom nginx template to set a temporary variable which will be used later

   The variable that the access log is reading from, needs to exist in the nginx.conf file prior to Kong starting, therefore a custom template is needed to set/create the variable first.

   In the Kong proxy block, we add the line: set $foo 'hellofromnginxtemplate';

   Example:

   Take Kong’s default Nginx template (kong/templates/nginx.lua, inlining the contents of kong/templates/nginx_kong.lua in place of its include 'nginx-kong.conf'; line, since that included file is otherwise always regenerated from the stock template and is not itself overridable) and add the set $foo ...; line inside the server { server_name kong; ... } block that handles the proxy listeners, for example:

   server {
       server_name kong;
       set $foo 'hellofromnginxtemplate';
   > for _, entry in ipairs(proxy_listeners) do
       listen $(entry.listener);
   > end
       ...

   Copied!

   Save the full, modified template to a file and start (or restart) Kong with kong start --nginx-conf /path/to/custom_nginx.template (or the equivalent KONG_NGINX_CONF/--nginx-conf mechanism for your deployment) so the variable exists before the access log format below references it.

2. Modify the access log format using the following kong.conf directives

   nginx_http_log_format="show_everything '\$time_iso8601 - \$bytes_sent - \$request - \$status - \$foo'"
   proxy_access_log=/dev/stdout show_everything

   Copied!

3. Restart Kong

4. Create a pre-function plugin and assign the following code to the access phase

   local kong = kong
   local ngx = ngx
   local route = kong.router.get_route()
   ngx.var.foo = route.name

   Copied!

Note: kong.router is only available under untrusted_lua = lax (or on) — Kong Gateway 3.14.0.0 defaults untrusted_lua to strict, under which kong.router is entirely absent from the sandboxed kong table, and the snippet above fails at request time with attempt to index field 'router' (a nil value) (a 500 to the client). Set untrusted_lua: lax (KONG_UNTRUSTED_LUA=lax) before creating this plugin, or the snippet above will not work.