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On Thu, Jan 28, 2010 at 6:31 PM, Qing Yan <qing...@gmail.com> wrote:

> Hi Qian Ye,
>
>
> Could you forward me a copy of the paper?  I don't have ACM access...duo
> xie!
>
>
> btw, I was a ZJUer too..
>
> cheers,
>
> Qing
>
>
>
> On Fri, Jan 29, 2010 at 10:02 AM, Qian Ye <yeqian....@gmail.com> wrote:
>
> > Thanks henry and ben, actually I have read the paper henry mentioned in
> > this
> > mail, but I'm still not so clear with some of the details. Anyway, maybe
> > more study on the source code can help me understanding. Since Ben said
> > that, "if less than a quorum of servers have accepted a transaction, we
> can
> > commit or discard". Would this feature cause any unexpected problem? Can
> > you
> > give some hints about this issue?
> >
> >
> >
> > On Fri, Jan 29, 2010 at 1:09 AM, Benjamin Reed <br...@yahoo-inc.com>
> > wrote:
> >
> > > henry is correct. just to state another way, Zab guarantees that if a
> > > quorum of servers have accepted a transaction, the transaction will
> > commit.
> > > this means that if less than a quorum of servers have accepted a
> > > transaction, we can commit or discard. the only constraint we have in
> > > choosing is ordering. we have to decide which partially accepted
> > > transactions are going to be committed and which discarded before we
> > propose
> > > any new messages so that ordering is preserved.
> > >
> > > ben
> > >
> > >
> > > Henry Robinson wrote:
> > >
> > >> Hi -
> > >>
> > >> Note that a machine that has the highest received zxid will
> necessarily
> > >> have
> > >> seen the most recent transaction that was logged by a quorum of
> > followers
> > >> (the FIFO property of TCP again ensures that all previous messages
> will
> > >> have
> > >> been seen). This is the property that ZAB needs to preserve. The idea
> is
> > >> to
> > >> avoid missing a commit that went to a node that has since failed.
> > >>
> > >> I was therefore slightly imprecise in my previous mail - it's possible
> > for
> > >> only partially-proposed proposals to be committed if the leader that
> is
> > >> elected next has seen them. Only when another proposal is committed
> > >> instead
> > >> must the original proposal be discarded.
> > >>
> > >> I highly recommend Ben Reed's and Flavio Junqueira's LADIS paper on
> the
> > >> subject, for those with portal.acm.org access:
> > >> http://portal.acm.org/citation.cfm?id=1529978
> > >>
> > >> Henry
> > >>
> > >> On 27 January 2010 21:52, Qian Ye <yeqian....@gmail.com> wrote:
> > >>
> > >>
> > >>
> > >>> Hi Henry:
> > >>>
> > >>> According to your explanation, "*ZAB makes the guarantee that a
> > proposal
> > >>> which has been logged by
> > >>> a quorum of followers will eventually be committed*" , however, the
> > >>> source
> > >>> code of Zookeeper, the FastLeaderElection.java file, shows that, in
> the
> > >>> election, the candidates only provide their zxid in the votes, the
> one
> > >>> with
> > >>> the max zxid would win the election. I mean, it seems that no check
> has
> > >>> been
> > >>> made to make sure whether the latest proposal has been logged by a
> > quorum
> > >>> of
> > >>> servers.
> > >>>
> > >>> In this situation, the zookeeper would deliver a proposal, which is
> > known
> > >>> as
> > >>> a failed one by the client. Imagine this scenario, a zookeeper
> cluster
> > >>> with
> > >>> 5 servers, Leader only receives 1 ack for proposal A, after a
> timeout,
> > >>> the
> > >>> client is told that the proposal failed. At this time, all servers
> > >>> restart
> > >>> due to a power failure. The server have the log of proposal A would
> be
> > >>> the
> > >>> leader, however, the client is told the proposal A failed.
> > >>>
> > >>> Do I misunderstand this?
> > >>>
> > >>>
> > >>> On Wed, Jan 27, 2010 at 10:37 AM, Henry Robinson <he...@cloudera.com
> >
> > >>> wrote:
> > >>>
> > >>>
> > >>>
> > >>>> Qing -
> > >>>>
> > >>>> That part of the documentation is slightly confusing. The elected
> > leader
> > >>>> must have the highest zxid that has been written to disk by a quorum
> > of
> > >>>> followers. ZAB makes the guarantee that a proposal which has been
> > logged
> > >>>>
> > >>>>
> > >>> by
> > >>>
> > >>>
> > >>>> a quorum of followers will eventually be committed. Conversely, any
> > >>>> proposals that *don't* get logged by a quorum before the leader
> > sending
> > >>>> them
> > >>>> dies will not be committed. One of the ZAB papers covers both these
> > >>>> situations - making sure proposals are committed or skipped at the
> > right
> > >>>> moments.
> > >>>>
> > >>>> So you get the neat property that leader election can be live in
> > exactly
> > >>>> the
> > >>>> case where the ZK cluster is live. If a quorum of peers aren't
> > available
> > >>>>
> > >>>>
> > >>> to
> > >>>
> > >>>
> > >>>> elect the leader, the resulting cluster won't be live anyhow, so
> it's
> > ok
> > >>>> for
> > >>>> leader election to fail.
> > >>>>
> > >>>> FLP impossibility isn't actually strictly relevant for ZAB, because
> > FLP
> > >>>> requires that message reordering is possible (see all the stuff in
> > that
> > >>>> paper about non-deterministically drawing messages from a
> potentially
> > >>>> deliverable set). TCP FIFO channels don't reorder, so provide the
> > extra
> > >>>> signalling that ZAB requires.
> > >>>>
> > >>>> cheers,
> > >>>> Henry
> > >>>>
> > >>>> 2010/1/26 Qing Yan <qing...@gmail.com>
> > >>>>
> > >>>>
> > >>>>
> > >>>>> Hi,
> > >>>>>
> > >>>>> I have question about how zookeeper *remembers* a commit operation.
> > >>>>>
> > >>>>> According to
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>
> > >>>
> >
> http://hadoop.apache.org/zookeeper/docs/r3.2.2/zookeeperInternals.html#sc_summary
> > >>>
> > >>>
> > >>>> <quote>
> > >>>>>
> > >>>>>
> > >>>>> The leader will issue a COMMIT to all followers as soon as a quorum
> > of
> > >>>>> followers have ACKed a message. Since messages are ACKed in order,
> > >>>>>
> > >>>>>
> > >>>> COMMITs
> > >>>>
> > >>>>
> > >>>>> will be sent by the leader as received by the followers in order.
> > >>>>>
> > >>>>> COMMITs are processed in order. Followers deliver a proposals
> message
> > >>>>>
> > >>>>>
> > >>>> when
> > >>>>
> > >>>>
> > >>>>> that proposal is committed.
> > >>>>> </quote>
> > >>>>>
> > >>>>> My question is will leader wait for COMMIT to be processed by
> quorum
> > >>>>> of followers before consider
> > >>>>> COMMIT to be success? From the documentation it seems that leader
> > >>>>>
> > >>>>>
> > >>>> handles
> > >>>
> > >>>
> > >>>> COMMIT asynchronously and
> > >>>>> don't expect confirmation from followers. In the extreme case, what
> > >>>>>
> > >>>>>
> > >>>> happens
> > >>>>
> > >>>>
> > >>>>> if leader issue a COMMIT
> > >>>>> to all followers and crash immediately before the COMMIT message
> can
> > go
> > >>>>>
> > >>>>>
> > >>>> out
> > >>>>
> > >>>>
> > >>>>> of the network. How the system
> > >>>>> remembers the COMMIT ever happens?
> > >>>>>
> > >>>>> Actually this is related to the leader election process:
> > >>>>>
> > >>>>> <quote>
> > >>>>> ZooKeeper messaging doesn't care about the exact method of electing
> a
> > >>>>> leader
> > >>>>> has long as the following holds:
> > >>>>>
> > >>>>>  -
> > >>>>>
> > >>>>>  The leader has seen the highest zxid of all the followers.
> > >>>>>  -
> > >>>>>
> > >>>>>  A quorum of servers have committed to following the leader.
> > >>>>>
> > >>>>>  Of these two requirements only the first, the highest zxid amoung
> > the
> > >>>>> followers needs to hold for correct operation.
> > >>>>>
> > >>>>> </quote>
> > >>>>>
> > >>>>> Is there a liveness issue try to find "The leader has seen the
> > highest
> > >>>>>
> > >>>>>
> > >>>> zxid
> > >>>>
> > >>>>
> > >>>>> of all the followers"? What if some of the followers (which happens
> > to
> > >>>>> holding the highest zxid) cannot be contacted(FLP impossible
> result?)
> > >>>>>  It will be more striaghtforward if COMMIT requires confirmation
> from
> > a
> > >>>>> quorum of the followers. But I guess things get
> > >>>>> optimized according to Zab's FIFO nature...just want to hear some
> > >>>>> clarification about it.
> > >>>>>
> > >>>>> Thanks alot!
> > >>>>>
> > >>>>>
> > >>>>>
> > >>>>
> > >>> --
> > >>> With Regards!
> > >>>
> > >>> Ye, Qian
> > >>> Made in Zhejiang University
> > >>>
> > >>>
> > >>>
> > >>
> > >>
> > >>
> > >>
> > >>
> > >
> > >
> >
> >
> > --
> >  With Regards!
> >
> > Ye, Qian
> > Made in Zhejiang University
> >
>



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