code: public int completed(InetAddress request)
      {
          String dc = DatabaseDescriptor.getEndpointSnitch().getDatacenter(request);
          Queue<InetAddress> requests = requestsByDatacenter.get(dc);
          assert requests != null;
          assert request.equals(requests.peek());
          requests.poll();
          if (!requests.isEmpty())
              processor.process(requests.peek());
          return --remaining;
      }
docs: Returns how many request remains