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From Jordan Zimmerman <jor...@jordanzimmerman.com>
Subject Re: locking/leader election and dealing with session loss
Date Wed, 15 Jul 2015 21:13:50 GMT
This property may hold if you make a lot of timing/synchrony assumptions
These assumptions and timing are intrinsic to using ZooKeeper. So, of course I’m making
these assumptions.


-Jordan



On July 15, 2015 at 3:57:12 PM, Alexander Shraer (shralex@gmail.com) wrote:

This property may hold if you make a lot of timing/synchrony assumptions -- agreeing on who
holds the lock in an asynchronous distributed system with failures is impossible, this is
the FLP impossibility. 

But even if it holds, this property is not very useful if the ZK client itself doesn't have
the application data. So one has to consider whether it is possible that the application sees
a messages from two clients that both think are the leader in an order which contradicts the
lock acquisition order.

On Wed, Jul 15, 2015 at 1:26 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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