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From Henry Robinson <he...@cloudera.com>
Subject Re: leader election, scheduled tasks, losing leadership
Date Sun, 09 Dec 2012 05:12:33 GMT
You can't assume that the notification is received locally before another
leader election finishes elsewhere (particularly if you are running slowly
for some reason!), so it's not sufficient to guarantee that the process
that is running locally has finished before someone else starts another.

It's usually best - if possible - to restructure the system so that
processes are idempotent to work around these kinds of problem, in
conjunction with using the kind of primitives that Curator provides.

Henry

On 8 December 2012 21:04, Jordan Zimmerman <jordan@jordanzimmerman.com>wrote:

> This is why you need a ConnectionStateListener. You'll get a notice that
> the connection has been suspended and you should assume all locks/leaders
> are invalid.
>
> -JZ
>
> On Dec 8, 2012, at 9:02 PM, Henry Robinson <henry@cloudera.com> wrote:
>
> > What about a network disconnection? Presumably leadership is revoked when
> > the leader appears to have failed, which can be for more reasons than a
> VM
> > crash (VM running slow, network event, GC pause etc).
> >
> > Henry
> >
> > On 8 December 2012 21:00, Jordan Zimmerman <jordan@jordanzimmerman.com
> >wrote:
> >
> >> The leader latch lock is the equivalent of task in progress. I assume
> the
> >> task is running in the same VM as the leader lock. The only reason the
> VM
> >> would lose leadership is if it crashes in which case the process would
> die
> >> anyway.
> >>
> >> -JZ
> >>
> >> On Dec 8, 2012, at 8:56 PM, Eric Pederson <ericacm@gmail.com> wrote:
> >>
> >>> If I recall correctly it was Henry Robinson that gave me the advice to
> >> have
> >>> a "task in progress" check.
> >>>
> >>>
> >>> -- Eric
> >>>
> >>>
> >>>
> >>> On Sat, Dec 8, 2012 at 11:54 PM, Eric Pederson <ericacm@gmail.com>
> >> wrote:
> >>>
> >>>> I am using Curator LeaderLatch :)
> >>>>
> >>>>
> >>>> -- Eric
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> On Sat, Dec 8, 2012 at 11:52 PM, Jordan Zimmerman <
> >>>> jordan@jordanzimmerman.com> wrote:
> >>>>
> >>>>> You might check your leader implementation. Writing a correct leader
> >>>>> recipe is actually quite challenging due to edge cases. Have a look
> at
> >>>>> Curator (disclosure: I wrote it) for an example.
> >>>>>
> >>>>> -JZ
> >>>>>
> >>>>> On Dec 8, 2012, at 8:49 PM, Eric Pederson <ericacm@gmail.com>
wrote:
> >>>>>
> >>>>>> Actually I had the same thought and didn't consider having to
do
> this
> >>>>> until
> >>>>>> I talked about my project at a Zookeeper User Group a month
or so
> ago
> >>>>> and I
> >>>>>> was given this advice.
> >>>>>>
> >>>>>> I know that I do see leadership being lost/transferred when
one of
> the
> >>>>> ZK
> >>>>>> servers is restarted (not the whole ensemble).   And it seems
like
> >> I've
> >>>>>> seen it happen even when the ensemble stays totally stable (though
I
> >> am
> >>>>> not
> >>>>>> 100% sure as it's been a while since I have worked on this
> particular
> >>>>>> application).
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> -- Eric
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On Sat, Dec 8, 2012 at 11:25 PM, Jordan Zimmerman <
> >>>>>> jordan@jordanzimmerman.com> wrote:
> >>>>>>
> >>>>>>> Why would it lose leadership? The only reason I can think
of is if
> >> the
> >>>>> ZK
> >>>>>>> cluster goes down. In normal use, the ZK cluster won't go
down (I
> >>>>> assume
> >>>>>>> you're running 3 or 5 instances).
> >>>>>>>
> >>>>>>> -JZ
> >>>>>>>
> >>>>>>> On Dec 8, 2012, at 8:17 PM, Eric Pederson <ericacm@gmail.com>
> wrote:
> >>>>>>>
> >>>>>>>> During the time the task is running a cluster member
could lose
> its
> >>>>>>>> leadership.
> >>>>>>>
> >>>>>>>
> >>>>>
> >>>>>
> >>>>
> >>
> >>
> >
> >
> > --
> > Henry Robinson
> > Software Engineer
> > Cloudera
> > 415-994-6679
>
>


-- 
Henry Robinson
Software Engineer
Cloudera
415-994-6679

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