NGinx : How to test if a cookie is set or not without using 'if'?

Question

I am using the following configuration for NGinx currently to test my app :

location / {
            # see if the 'id' cookie is set, if yes, pass to that server.           
            if ($cookie_id){
                proxy_pass http://${cookie_id}/$request_uri;                
                break;
            }

            # if the cookie isn't set, then send him to somewhere else
            proxy_pass http://localhost:99/index.php/setUserCookie;
        }

But they say "IFisEvil". Can anyone show me a way how to do the same job without using "if"?
And also, is my usage of "if" is buggy?

Answer 1

There are two reasons why 'if is evil' as far as nginx is concerned. One is that many howtos found on the internet will directly translate htaccess rewrite rules into a series of ifs, when separate servers or locations would be a better choice. Secondly, nginx's if statement doesn't behave the way most people expect it to. It acts more like a nested location, and some settings don't inherit as you would expect. Its behavior is explained here.

That said, checking things like cookies must be done with ifs. Just be sure you read and understand how ifs work (especially regarding directive inheritance) and you should be ok.

You may want to rethink blindly proxying to whatever host is set in the cookie. Perhaps combine the cookie with a map to limit the backends.

EDIT: If you use names instead of ip addresses in the id cookie, you'll also need a resolver defined so nginx can look up the address of the backend. Also, your default proxy_pass will append the request onto the end of the setUserCookie. If you want to proxy to exactly that url, you replace that default proxy_pass with:

rewrite ^ /index.php/setUserCookie break;
proxy_pass http://localhost:99;