Nginx upstream as a variable

Upstream wdzjbbs_varnish{
Session_sticky;
Server 10.174.35.11 weight=1 max_fails=2 fail_timeout=30s;
}

Upstream wdzjbbs{
Session_sticky;
Server 10.174.39.137 weight=1 max_fails=2 fail_timeout=30s;
}

server {

...

Set $upstream _x "Wdzjbbs_varnish";
if ($http _cookie ~* "Auth_token") {
Set $upstream _x "Wdzjbbs";
}
if ($request _uri ~* "search") {
Set $upstream _x "Wdzjbbs";

}

...

Location/{
Proxy_pass http://$upstream _x;
Proxy_set_header Host $host;
Proxy_set_header X-real-ip $remote _addr;
Proxy_set_header x-forwarded-for $proxy _add_x_forwarded_for;

}

Note: Using the above to determine the Upsteram variable, this can be done by the cookie to determine if there is no back jump to the background which server

Nginx upstream as a variable