def get_canonical_key_id(self, key_id):
        """
        get_canonical_key_id is used by get_canonical_key, see the comment
        for that method for more explanation.

        Keyword arguments:
        key_id -- the key id (e.g. '12345')

        returns the canonical key id (e.g. '12')
        """
        shard_num = self.get_shard_num_by_key_id(key_id)
        return self._canonical_keys[shard_num]
get_canonical_key_id is used by get_canonical_key, see the comment
        for that method for more explanation.

        Keyword arguments:
        key_id -- the key id (e.g. '12345')

        returns the canonical key id (e.g. '12')