def watch(self, key, pipeline=False):
        """Watch the given key.

        Marks the given key to be watch for conditional execution
        of a transaction.

        Args:
            key (str): Key that needs to be watched
            pipeline (bool): True, start a transaction block. Default false.

        """
        if pipeline:
            self._pipeline.watch(key)
        else:
            self._db.watch(key)