Re: Use of mutex in persistency store

Wouter,

On 10/28/2013 07:58 PM, Wouter Beek wrote:
> Hi,
>
> In the documentation for the persistency
> library<http://www.swi-prolog.org/pldoc/doc/home/vnc/prolog/lib/swipl/library/persistency.pl>,
> the following code occurs:
> ~~~{.pl}
> current_user_role(Name, Role) :-
>    with_mutex(user_db, user_role(Name, Role)).
> ~~~
> I do not get the use of with_mutex/1 here. Could reading from the
> persistency store cause troubles in another thread, and if so how?

Suppose another thread calls set_user_role(+Name, +Role) which does
a retractall and assert.  Now, there is a point in time where there
is no user role defined and user_role/2 will fail.  If you swap the
assert and retract, there is a point in time that there are two roles.

The clean solution is a transactional Prolog database as I described
for CICLOPS last year.  Without, you need both a mutex to set and to
retrieve the current value.  Mutexes for retrieving values 
deterministically isn't too bad, but non-deterministically it is easy
to have situations where the lock is held long.

	Cheers --- Jan


>
> ---
> Cheers!,
> Wouter.
>
> E-mail: [email protected]
> WWW: www.wouterbeek.com
> Tel.: 0647674624
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