Use varnish's round-robin director instead of random director to avoi…

…d getting 503 errors when some (but not all) back-ends are down. (The random director doesn't necessarily try all back-ends, even if retries is set >= than the number of back-ends.)

BTW this is only for balancing between back-end nginx servers and there's no problem doing round-robin here, the web servers themselves are still using Passenger's global queueing to balance between the Rails back-ends so there is no issue with one slow request holding up all the others.

server/files/etc/varnish/default.vcl.erb

@@ -1,8 +1,7 @@
 #This is the VCL configuration file for varnish.  See the vcl(7)
 #man page for details on VCL syntax and semantics.
 
-director director_1 random {
-  .retries = <%= roles[:web].size * 3 %>;
+director director_1 round-robin {
   # One backend for each instance in the "web" role. If that instance is also in
   # the "proxy" role then it's listening on port 81, otherwise it's on port 80
   <% roles[:web].each_with_index do |address, i| %>
@@ -12,10 +11,9 @@ director director_1 random {
         .port = "<%= roles[:proxy] && roles[:proxy].include?(address) ? 81 : 80  %>";
         .max_connections = 100;
         .connect_timeout = 10s;
-        .first_byte_timeout = 60s;
+        .first_byte_timeout = 30s;
         .between_bytes_timeout = 10s;
       }
-      .weight = 1;
     }
   <% end %>
 }