deploy: allow clients to override how they are routed with route_override cookie (#1583)

Author: dyc3

deploy/nginx.prod.conf

@@ -17,6 +17,13 @@ map $http_upgrade $connection_upgrade {
 	'' close;
 }
 
+# this allows us to override the upstream based on a cookie called `route_override`
+map $cookie_route_override $upstream_override {
+	default $split_upstream;
+	monolith _monolith;
+	balancer _balancer;
+}
+
 proxy_cache_path /var/cache/nginx levels=1:2 keys_zone=one:8m max_size=1000m inactive=600m;
 
 server {
@@ -25,7 +32,7 @@ server {
 	http2 on;
 
 	location / {
-		proxy_pass http://$split_upstream$request_uri;
+		proxy_pass http://$upstream_override$request_uri;
 		proxy_http_version 1.1;
 		proxy_set_header Upgrade $http_upgrade;
 		proxy_set_header Connection "upgrade";
@@ -42,5 +49,7 @@ server {
 		proxy_send_timeout 7d;
 
 		proxy_socket_keepalive on;
+
+		add_header X-Upstream $upstream_override;
 	}
 }