Nginx | How to configure based on urls & header

Question

Basically i have 2 rules already based on url pattern. Now i would like add another condition based on header value, let say the if source header = "gif" then redirect the request to this upstream "http://zone3.svc.cluster.local:4911;", if the source present but the value is different, if source is not even present then proceed with url based conditions i have below and redirect the request. How can achieve this using nginx ? Nginx version: v1.14.2

location ~ ^/v[1-9]+(.[0-9]+)*/abc/fetch {
set $upstream http://zone1.svc.cluster.local:4911;
proxy_pass $upstream;
}

location ~ ^/v[1-9]+(.[0-9]+)*/abc/b2c {
set $upstream http://zone2.svc.cluster.local:4911;
proxy_pass $upstream;
}

// fallback rule
location ~ ^/v[1-9]+(.[0-9]+)*/abc {
set $upstream http://zone.svc.cluster.local:4911;
proxy_pass $upstream;
}

There is also buzz around not use if, as if being evil in nginx. What way is better.

Answer 1

This should work. I got this from nginxcommunity slack. Person Name: Liam.

map $http_source $final_upstream {
   "gif" http://zone3.svc.cluster.local:4911;
   "png" http://zone3.svc.cluster.local:4912;
    default $upstream;
}

server {
    location ~ ^/v[1-9]+(.[0-9]+)*/abc/fetch {
        set $upstream http://zone1.svc.cluster.local:4911;
        proxy_pass $final_upstream; # Evaluated by map block
    }
    #...
}