duplicating or forking incoming requests

Gary Foster gfoster at realgravity.com
Fri Jul 12 19:47:02 UTC 2013

I'm trying to figure out how to accomplish something with nginx and it seems to have me baffled.

We use nginx as an endpoint to log incoming events.  The default response is a 200, empty body and an entry in the access log in a very specific format.  For legacy reasons, this will not change any time soon.

Now, in addition to just simply logging this request, we want to forward that request to an internal server to also do something with it (and again, for legacy reasons I can't just make that server do the logging instead).

I can get this working fine, except for failure modes… I specifically do not want the original nginx logging affected at all if it can't proxy the request upstream, and in point of fact specifically want to return a 200 in that case also.  So basically the desired behavior looks like this:

nginx running
proxy running
log a successful incoming GET request to nginx access log
forward request to proxy
return 200

nginx running
proxy not running (or returns an error)
log a successful incoming GET request to nginx access log
forward request to proxy
proxy returns an error
nginx ignores the error and returns a 200

I haven't been able to get to that point, but instead have been only able to get various combinations of "nginx doesn't log anything", "nginx logs everything only when the entire chain is up" and "nginx logs incoming requests fine when chain is up but logs to the error log when the proxy is down".

Here's the configuration snippets I'm currently using:

  location = /events {
    access_log events_access.log events_format;
    expires 1s;
    try_files @proxy @proxy;
    # try_files @proxy /empty.html;
    # try_files @proxy =200;

  location @proxy {
    proxy_intercept_errors on;
    access_log events_access.log events_format;
    error_page 502 =200 /empty.html;
    proxy_set_header X-Real-IP $remote_addr;

Basically, what I want is that if it logs every incoming request normally.  If it can forward the request to the upstream proxy, it does so after logging it, and if it can't, it simply logs it and returns a 200.

Is this possible and if so how?

Thanks in advance!

-- Gary F.

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