def sortKey():
    """Sort key used to order distributed transactions

        When a transaction involved multiple storages, 2-phase commit
        operations are applied in sort-key order.  This must be unique
        among storages used in a transaction. Obviously, the storage
        can't assure this, but it should construct the sort key so it
        has a reasonable chance of being unique.

        The result must be a string.
        """
