async def on_connect(self):
        """
        Initialize the connection, authenticate and select a database and send READONLY if it is
        set during object initialization.
        """
        if self.db:
            warnings.warn('SELECT DB is not allowed in cluster mode')
            self.db = ''
        await super(ClusterConnection, self).on_connect()
        if self.readonly:
            await self.send_command('READONLY')
            if nativestr(await self.read_response()) != 'OK':
                raise ConnectionError('READONLY command failed')
Initialize the connection, authenticate and select a database and send READONLY if it is
        set during object initialization.