Setting a trace id in nginx load balancer

Question

I'm using nginx as a load balancer in front of several upstream app servers and I want to set a trace id to use to correlate requests with the app server logs. What's the best way to do that in Nginx, is there a good 3rd party module for this?

Otherwise a pretty simple way would be to base it off of timestamp (possibly plus a random number if that's not precise enough) and set it as an extra header on the request, but the only set_header command I see in the docs is for setting a response header.

Answer 1

In most cases you don't need a custom module, you can simply set a header with a combination of embedded variables of http_core_module which is (most probably) unique. Example:

  location / {
      proxy_pass http://upstream;
      proxy_set_header X-Request-Id $pid-$msec-$remote_addr-$request_length;
  }

This would yield a request id like "31725-1406109429.299-127.0.0.1-1227" and should be "unique enough" to serve as a trace id.