def sync_proxy(self, mri, block):
        """Abstract method telling the ClientComms to sync this proxy Block
        with its remote counterpart. Should wait until it is connected

        Args:
            mri (str): The mri for the remote block
            block (BlockModel): The local proxy Block to keep in sync
        """
        # Send a root Subscribe to the server
        subscribe = Subscribe(path=[mri], delta=True)
        done_queue = Queue()

        def handle_response(response):
            # Called from tornado
            if not isinstance(response, Delta):
                # Return or Error is the end of our subscription, log and ignore
                self.log.debug("Proxy got response %r", response)
                done_queue.put(None)
            else:
                cothread.Callback(
                    self._handle_response, response, block, done_queue)

        subscribe.set_callback(handle_response)
        IOLoopHelper.call(self._send_request, subscribe)
        done_queue.get(timeout=DEFAULT_TIMEOUT)