[REBOL] Re: Multiuser database
From: SunandaDH:aol at: 25-Mar-2006 17:18
> Paavo:
> I have been quite happy with semaphores.
Sunanda:
> It'd be worth checking the source of the semaphore handler you are using.
Paavo sent me the source of the functions he'd been using. They'd come
originally from Brett's obtain-semaphore and release-semaphore:
http://www.codeconscious.com/rebol/rebol-net.html
Brett's come up with a neater idea than trying to open a file for update to
obtain an exclusive lock. He's flipping a file between two names.
That should result in an atomic operation on pretty much all platforms, so
could be used as a fairly safe locking mechanism when all you have in common is
access to a shared folder.
It has problems as noted earlier in this thread -- if the task that applies
the lock dies, then all future tasks are blocked. It also requires some sort
of single-threaded start-up to create the file in the unlocked state; and if
that start-up process is accidentally run again, it could accidentally release a
lock.
Those problems are fixable at the cost of a greatly complicated lock protocol.
As Gabriele has said, this stuff isn't easy. Or at least, it isn't easy if
you are trying to do it from outside of a privileged kernel state.
Sunanda.
