Some good points in that article, but it overlooks some important aspects.
In a nutshell:
- It only covers container-level optimistic concurrency. The alternative
(database-level optimistic concurrency) has its uses as well
- The two-phase SELECT cancels most of the benefits of optimistic
concurrency, and it penalizes read-only clients. If you choose to go for
optimistic concurrency in the first place, you want to be... well...
optimistic. You believe that most of the time, the data you are updating
has *not* been modified, and therefore, that a "last minute check" is
unnecessary, or even unacceptable performance-wise.
But the two-phase SELECT imposes an overhead on *all* EJB's and *all* their
transactions. In that respect, it is a half-baked solution, a choice you
should make when you don't know exactly the ratio of writes/reads that will
happen on your rows. You don't want to go Pessimistic, but you're afraid
that being overly Optimistic will result in too many rollbacks.
No silver bullet there, just trade-offs.
How optimistic do you want to be today? ;-)
--
Cedric
> -----Original Message-----
> From: A mailing list for Enterprise JavaBeans development
> [mailto:[EMAIL PROTECTED]]On Behalf Of Jonathan K. Weedon
> Sent: Wednesday, June 27, 2001 2:06 PM
> To: [EMAIL PROTECTED]
> Subject: Re: Are Entity Beans(CMP/BMP) really necessary???
>
>
> For readers who are interested in this topic, I recommend the
> thread titled:
>
> Real-world limitations of simplistic optimistic concurrency
>
> Which can be found at:
>
>
> http://archives.java.sun.com/cgi-bin/wa?A2=ind0009&L=ejb-interest&;
> F=&S=&P=113913
>
> -jkw
>
> "Jonathan K. Weedon" wrote:
>
> > Cedric,
> >
> > Actually, I believe you provide the following:
> >
> > Exclusive concurrency (pessimistic locking at the container level)
> >
> > This cannot be used in conjunction with clustering, and is therefore
> > an unscalable solution.
> >
> > Database based pessimistic concurrency
> >
> > This can be used with clustering, but is not scalable due to excessive
> > locking in the database.
> >
> > Database based optimistic concurrency
> >
> > This corrupts the database, if done without verified updates (this is
> > the point that you made yourself, originally).
> >
> > So, you have a choice between unscalable (pessimistic locking, either
> > in an unclustered container, or in the central database) or incorrect
> > (due to unverified updates).
> >
> > If there is a better option, please explain it. In particular, how do I
> > deploy a scalable application (meaning one that uses optimistic
> > concurrency) on WebLogic without running the risk of corrupting
> > my database (due to a lack of verified updates).
> >
> > -jkw
> >
> > Cedric Beust wrote:
> >
> > > > From: A mailing list for Enterprise JavaBeans development
> > > > [mailto:[EMAIL PROTECTED]]On Behalf Of Jonathan K. Weedon
> > >
> > > > Is such a feature provided in WL6 using EJB 1.1? I did not
> > > > believe so.
> > >
> > > No, we provide either Exclusive concurrency (pessimistic at
> the container
> > > level) or Database (pushed to the database, which can be
> either optimistic
> > > or pessimistic depending on your database vendor).
> > >
> > > None of them corrupt the database (that I know :-)).
> > >
> > > --
> > > Cedric
> > >
> > >
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> > --
> > Jonathan K. Weedon: Architect
> > VisiBroker, Borland AppServer
> >
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> Jonathan K. Weedon: Architect
> VisiBroker, Borland AppServer
>
> Register now for the 12th Annual Borland Conference: July 21-25 in
> Long Beach, California. BorCon is the best place to learn about award
> winning technologies for implementing e-business. Register today!
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