def start_proxy(self):
        """
        Starts Cloud SQL Proxy.

        You have to remember to stop the proxy if you started it!
        """
        self._download_sql_proxy_if_needed()
        if self.sql_proxy_process:
            raise AirflowException("The sql proxy is already running: {}".format(
                self.sql_proxy_process))
        else:
            command_to_run = [self.sql_proxy_path]
            command_to_run.extend(self.command_line_parameters)
            try:
                self.log.info("Creating directory %s",
                              self.cloud_sql_proxy_socket_directory)
                os.makedirs(self.cloud_sql_proxy_socket_directory)
            except OSError:
                # Needed for python 2 compatibility (exists_ok missing)
                pass
            command_to_run.extend(self._get_credential_parameters())
            self.log.info("Running the command: `%s`", " ".join(command_to_run))
            self.sql_proxy_process = Popen(command_to_run,
                                           stdin=PIPE, stdout=PIPE, stderr=PIPE)
            self.log.info("The pid of cloud_sql_proxy: %s", self.sql_proxy_process.pid)
            while True:
                line = self.sql_proxy_process.stderr.readline().decode('utf-8')
                return_code = self.sql_proxy_process.poll()
                if line == '' and return_code is not None:
                    self.sql_proxy_process = None
                    raise AirflowException(
                        "The cloud_sql_proxy finished early with return code {}!".format(
                            return_code))
                if line != '':
                    self.log.info(line)
                if "googleapi: Error" in line or "invalid instance name:" in line:
                    self.stop_proxy()
                    raise AirflowException(
                        "Error when starting the cloud_sql_proxy {}!".format(
                            line))
                if "Ready for new connections" in line:
                    return
Starts Cloud SQL Proxy.

        You have to remember to stop the proxy if you started it!