def dump_begin(self, selector_id):
        """Start dumping a stream.

        Args:
            selector_id (int): The buffered stream we want to dump.

        Returns:
            (int, int, int): Error code, second error code, number of available readings
        """

        if self.dump_walker is not None:
            self.storage.destroy_walker(self.dump_walker)

        selector = DataStreamSelector.FromEncoded(selector_id)
        self.dump_walker = self.storage.create_walker(selector, skip_all=False)

        return Error.NO_ERROR, Error.NO_ERROR, self.dump_walker.count()