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From John Wilson <sami.hailu...@gmail.com>
Subject Re: Optimistic Locking and the Prepare Phase
Date Tue, 13 Feb 2018 14:10:45 GMT
Hi  Vladimir,

Your answer is what is depicted in the graphics and makes perfect sense to
me. I guess what I'm confused about is what a "prepare" phase means and
what "*In optimistic transactions, locks are acquired on primary nodes
during the "prepare" phase* " means.

My understanding of a "prepare" phase based on the blog here (
https://www.gridgain.com/resources/blog/apache-ignite-transactions-architecture-2-phase-commit-protocol)
is that it is the phase where we acquire all the necessary locks (in
pessimistic locking) before we start the commit phase.


   1. In the context of *pessimistic* locking, at the end of the prepare
   phase but before we start commit, we would have acquired all locks. True?
   2. In the context of *optimistic* locking, a prepare phase would not
   request for or acquire locks. True?
   3. In the context of *optimistic* locking, at the end of the prepare
   phase but before we start commit, we have stored the current version of the
   keys in the transaction coordinator but we have not yet requested or
   acquired any locks. Locks will be acquired during the commit phase. True?

Thanks!

On Tue, Feb 13, 2018 at 12:54 AM, Vladimir Ozerov <vozerov@gridgain.com>
wrote:

> Hi John,
>
> 1) In PESSIMISTIC mode locks are obtained either on first update
> (READ_COMMITTED) or even read (REPEATABLE_READ). I.e. they obtained before
> prepare phase and are held for the duration of transaction. In OPTIMISTIC
> mode locks are obtained only after you call IgniteTransaction.commit().
> 2) It means that transaction will fail if enlisted entries have been
> changed after they were accessed by current transaction, but before this
> transaction is committed.
>
> On Tue, Feb 13, 2018 at 9:49 AM, John Wilson <sami.hailu.15@gmail.com>
> wrote:
>
> > Hi,
> >
> > The design doc below states:
> >
> > *" In optimistic transactions, locks are acquired on primary nodes during
> > the "prepare" phase, then promoted to backup nodes and released once the
> > transaction is committed. Depending on an isolation level, if Ignite
> > detects that a version of an entry has been changed since the time it was
> > requested by a transaction, then the transaction will fail at the
> "prepare"
> > phase and it will be up to an application to decide whether to restart
> the
> > transaction or not."*
> >
> > Two questions:
> >
> >
> >    1. If locks are acquired during the prepare phase, why do we state
> that
> >    lock acquisition for optimistic locking is delayed (as compared
> against
> >    pessimistic locking)?
> >    2. If "*ignite detects the version has changed since last request by
> >    transaction, it will fail at prepare phase*". Very confusing. What is
> >    the last request? I thought the "last request" means the "prepare"
> phase
> >    and if so why we say it may fail during prepare phase?
> >
> > The graphic make sense to me - i.e. locks for optimistic locking are
> > acquired on the commit phase and not on the prepare phase.
> >
> > https://cwiki.apache.org/confluence/display/IGNITE/
> > Ignite+Key-Value+Transactions+Architecture
> >
> > Please help clarify.
> >
> > Thanks.
> >
>

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