Unique ID for request tracing in Nginx

Question

We are running Nginx in a reverse proxy configuration, and I need to trace requests end-to-end. With version 1.11 Nginx introduced a new variable $request_id, but unfortunately we're still using version 1.10, so a config check reports

nginx: [emerg] unknown "request_id" variable
nginx: configuration file /etc/nginx/nginx.conf test failed

Can I use something else instead?

Answer 1

You can define a custom variable in your vhost where you combine other variables to a unique string, for instance:

set $my_req_id $connection-$connection_requests-$msec;

• $connection: serial number of the connection
• $connection_requests: current number of requests made through the connection
• $msec: current time in seconds with milliseconds resolution

Then set a custom header using that variable:

add_header X-Request-Id $my_req_id;          # for responses sent to the client
location / {
  proxy_set_header X-Request-Id $my_req_id;  # for requests sent to upstream backend
}

Reload or restart Nginx and you should see the new header appear in requests/responses.