Re: The Practical Benefits of the Relational Model

"Jan.Hidders" <hidders_at_hcoss.uia.ac.be> wrote in message news:3dafc37f$1_at_news.uia.ac.be...
> In article <aon0qq$1jak$1_at_sp15at20.hursley.ibm.com>,
> Paul Vernon <paul.vernon_at_ukk.ibmm.comm> wrote:
> >
> >"Jan.Hidders" <hidders_at_hcoss.uia.ac.be> wrote in message
> >news:3dadc2d1$1_at_news.uia.ac.be...
> >> >The only things they provide that cannot be done using core model
> >> >concepts, are things that explicitly do not want to be in the hands of
> >> >users.
> >>
> >> What they provide is the illusion that you are the only one who is
> >> allowed to use the database at the moment and everybody else has to
wait.
> >
> >At the cost of other users having to wait.
>
> No, the *illusion* that the other users have to wait. How much the other
> users really have to wait depends upon the locking protocol.

And on how trustworthy the other users are.

> >Optimistic locking does not provide this illusion.
>
> What!? Are you seriously suggesting that optimistic locking protocols
cannot
> guarantee serializability?

No. I'm suggesting that no locking protocol can give the "illusion that you are the only one who is allowed to use the database at the moment" because if I'm waiting (more than a second or two) on another (untrustworthy) user for my transaction to complete, then I don't have the illusion that I'm the only one allowed to use the database at any moment I choose.

To repeat. If you cannot trust your users then the concept of user visible transactions is wrong.

Regards
Paul Vernon
Business Intelligence, IBM Global Services Received on Fri Oct 18 2002 - 12:34:04 CEST