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From Ivan Kelly <iv...@apache.org>
Subject Re: locking/leader election and dealing with session loss
Date Wed, 15 Jul 2015 20:54:04 GMT
Jordan, imagine you have a node which is leader using the hbase example. A
client makes some request to the leader, which processes the request, lines
up a write to the state in hbase, and promptly goes into a 30 second gc
pause just before it flushes the socket. During the 30 second pause another
node takes over as leader and starts writing to the state. Now, when the
pause ends, what will stop the write from the first leader being flushed to
the socket and then hitting hbase?

-Ivan

On Wed, Jul 15, 2015 at 10:26 PM Jordan Zimmerman <
jordan@jordanzimmerman.com> wrote:

> I think we may be talking past each other here. My contention (and the ZK
> docs agree BTW) is that, properly written and configured, "at any
> snapshot in time no two clients think they hold the same lock”. How your
> application acts on that fact is another thing. You might need sequence
> numbers, you might not.
>
> -Jordan
>
>
> On July 15, 2015 at 3:15:16 PM, Alexander Shraer (shralex@gmail.com)
> wrote:
>
> Jordan, as Camille suggested, please read Sec 2.4 in the Chubby paper:
> link
> <
> http://static.googleusercontent.com/media/research.google.com/en//archive/chubby-osdi06.pdf>
>
>
> it suggests 2 ways in which the storage can support lock generations and
> proposes an alternative for the case where the storage can't be made aware
> of lock generations.
>
> On Wed, Jul 15, 2015 at 1:08 PM, Jordan Zimmerman <
> jordan@jordanzimmerman.com> wrote:
>
> > Ivan, I just read the blog and I still don’t see how this can happen.
> > Sorry if I’m being dense. I’d appreciate a discussion on this. In your
> blog
> > you state: "when ZooKeeper tells you that you are leader, there’s no
> > guarantee that there isn’t another node that 'thinks' its the leader.”
> > However, given a long enough session time — I usually recommend 30–60
> > seconds, I don’t see how this can happen. The client itself determines
> that
> > there is a network partition when there is no heartbeat success. The
> > heartbeat is a fraction of the session timeout. Once the heartbeat
> fails,
> > the client must assume it no longer has the lock. Another client cannot
> > take over the lock until, at minimum, session timeout. So, how then can
> > there be two leaders?
> >
> > -Jordan
> >
> > On July 15, 2015 at 2:23:12 PM, Ivan Kelly (ivank@apache.org) wrote:
> >
> > I blogged about this exact problem a couple of weeks ago [1]. I give an
> > example of how split brain can happen in a resource under a zk lock
> (Hbase
> > in this case). As Camille says, sequence numbers ftw. I'll add that the
> > data store has to support them though, which not all do (in fact I've
> yet
> > to see one in the wild that does). I've implemented a prototype that
> works
> > with hbase[2] if you want to see what it looks like.
> >
> > -Ivan
> >
> > [1]
> >
> >
> https://medium.com/@ivankelly/reliable-table-writer-locks-for-hbase-731024295215
> > [2] https://github.com/ivankelly/hbase-exclusive-writer
> >
> > On Wed, Jul 15, 2015 at 9:16 PM Vikas Mehta <vikasmehta@gmail.com>
> wrote:
> >
> > > Jordan, I mean the client gives up the lock and stops working on the
> > shared
> > > resource. So when zookeeper is unavailable, no one is working on any
> > shared
> > > resource (because they cannot distinguish network partition from
> > zookeeper
> > > DEAD scenario).
> > >
> > >
> > >
> > > --
> > > View this message in context:
> > >
> >
> http://zookeeper-user.578899.n2.nabble.com/locking-leader-election-and-dealing-with-session-loss-tp7581277p7581293.html
> > > Sent from the zookeeper-user mailing list archive at Nabble.com.
> > >
> >
>
>

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