code: async def on_connect(self):
          
          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')
docs: Initialize the connection, authenticate and select a database and send READONLY if it is
          set during object initialization.