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From Maciej <jezd...@gmail.com>
Subject Re: Ensure there is one master
Date Tue, 26 Nov 2013 23:17:25 GMT
Old master in part B will notice that the connection is broken after some time.
New master can be elected in part A earlier.

Let.s say that the system uses a shared disk (SAN).
There is some area on the disk that must be used exclusively (let's
name it EXC).

During this short period of time:

new master will receive a user request, will modify EXC area, and reply to user
old master will also receive another user request, will read EXC data
(during modification by new master) - reading inconsistent data, and
will reply to user - with inconsistent data

I wonder how to avoid such inconsistency.

On 11/26/13, Cameron McKenzie [via zookeeper-user]
<ml-node+s578899n7579377h23@n2.nabble.com> wrote:
> If I'm understanding your question correctly, you're worried that when the
> current 'master' loses its connection to ZooKeeper, a new 'master' will be
> elected and you will have 2 'master' nodes at the same time. As soon as you
> lose a connection to ZooKeeper there are no guarantees about any of the
> state that you're determining from it. When you lose the ZooKeeper
> connection, your 'master' must assume that it is no longer a 'master' node
> until it reconnects to ZooKeeper, at which point it will be able to work
> out what's going on.
> If you look at Apache Curator, its implementation of the Leader latch
> recipe handles this loss of connection and reestablishment.
> cheers
> Cam
> On Wed, Nov 27, 2013 at 9:28 AM, ms209495 <jezdnia@gmail.com> wrote:
>> Thanks for the reply. I want to clarify one thing.
>> I think about a System of 20 nodes, that uses ZooKeeper of 3 nodes.
>> I think about master election among these 20 nodes, that do not run
>> consensus, but they use zookeeper service for master election.
>> I used 'leader' term for a leeder in Zookeeper (among 3 nodes), and
>> 'master'
>> term for master in the System (20 nodes).
>> Solution is described here:
>> http://zookeeper.apache.org/doc/trunk/recipes.html#sc_leaderElection (I
>> would name it 'master' election, not 'leader' election), but I doubt if
>> it
>> works reliable without additional timing assumptions as I described in my
>> previous post.
>> Please consider my previous post in the context of the System that uses
>> Zookeeper (not ZooKeeper itself).
>> --
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