pretty much the same way a push load balancer does https://github.com/jackdoe/back-to-back/blob/master/broker/b...

    for {
      select {
      case reply := <-replyChannel:
       if reply.Uuid == r.message.Uuid {
        return reply
       }
      case <-timeout:
       return makeError(r.message, MessageType_ERROR_CONSUMER_TIMEOUT, "consumer timed out")
      }
     }

not much different than what you do with normal http timeouts, you send a request, sometimes a response comes sometimes it doesnt, up to the load balancer to decide if it wants to retry or error out

also, queue does not mean async model, it means a queue, there are many queues in http requests responses (e.g. the listen(2) backlog queue itself) and it does not make it async :)

EDIT: i dont have reply button to your 'what if there is modified state' question, but the answer is: exactly the same as push load balancing when you dont receive the instance's response due to network failure, but it modified the state