Re: computational model of transactions

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:EoJAg.34255$pu3.448878_at_ursa-nb00s0.nbnet.nb.ca...
> Marshall wrote:
>
> > Brian Selzer wrote:
> >
> >>"Erwin" <e.smout_at_myonline.be> wrote in message
> >>
> >>>Since each of them wants to update the very same resource (the same
> >>>attribute of the same tuple of the same relvar), these transactions
> >>>should be serialized anyway.
> >>
> >>I disagree. It is not always the case that if more than one actor is
> >>updating the same resource, that those updates must be serialized. To
> >>illustrate this, ignore the business rules in the above example. The
> >>semantics of the update involve modification, not replacement, the
operation
> >>involved, addition, is communitive and associative, and all of the
updates
> >>in question have the same semantics; therefore, it is only important
that
> >>the modifications be either disjoint or aggregated, but not necessarily
> >>serialized: the order in which the modifications occur is not important,
> >>since the end result is the same. In addition, only the operation on
the
> >>shared resource need be disjoint, other operations involving other
resources
> >>within each transaction can occur simultaneously, so serializing each
entire
> >>transaction would be overkill.
> >
> > This assumes the existence of an atomic addition operator;
> > no such operator exists as far as I know. It is always
> > read-add-write.
>
> Whether it is atomic depends on the hardware architecture just as it
> does for test-and-set.
>
>
> Given that read and write occur at separate
> > times, a race condition exists. *Something* has to be done
> > about the race condition, whatever the logical model.
> >
> > What you say above would make sense in the presence of
> > an atomic "+=" operator a la C's +=. But it has to be atomic.
>
> If I recall correctly, which is highly suspect, the PDP-11 can add to a
> word of memory.

I believe you are correct. whether or not a single machine instruction could be interrupted "in the middle" or not is something I wouldn't know.

Back in the day, I programmed a PDP-10, which was a one address machine. Received on Fri Aug 04 2006 - 19:02:24 CEST