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Date: Wed, 6 Nov 2013 07:44:10 +1100
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 <CANykduZ1D+1i638uGpK94sZZULVWxCv2zE5ivtgtFAf3Uqku7A@mail.gmail.com>
Subject: Re: Running Zookeeper in 2 machines
From: Cameron McKenzie <mckenzie.cam@gmail.com>
To: user@zookeeper.apache.org
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I have a similar problem to you. I have more than 2 machines, but only 2
geographically redundant sites.

In your situation, you could get some redundancy by running 2 instances on
one host, and 1 instance on the other host. This would protect you from
temporary network glitches (because the machine with 2 instances can still
form a quorum), and will protect you from failure of the machine with the
single instance. It will not help you if the machine with 2 instances
crashes.

In this situation, where the 2 instance machine dies, you can temporarily
configure the 1 instance machine to be a single instance cluster, and then
when the 2 instance machine is recovered, you can reconfigure the single
instance machine to be part of the 3 instance cluster again. This process
is manual, and slightly dangerous, because if you restart nodes in the
wrong order, you have potential to lose data. This is the approach that I
have tested and seems to work, but I'd recommend testing it also.

Machine A has ZK instance 1
Machine B has ZK instances 2 and 3

Machine B dies
Reconfigure ZK instance 1 so that it only has itself in the cluster. This
means that there is no redundancy at this point, but it can form a quorum
as its the only instance in the cluster.
Restart ZK instance 1 to pickup config changes
Fix up Machine B
Reconfigure ZK 1 instance to have ZK instances 2 and 3 in its configuration
Restart ZK instance 1 to pickup config changes
Start ZK instance 2 on Machine B.
Wait for ZK instance 1 on Machine A and ZK instance 2 on machine B form a
quorum. This is vitally important. If you start instance 3 before a quorum
is formed it is possible that instances 2 and 3 will form a quorum. This
will cause any updates that have occurred via instance 1 during the outage
of Machine B to be lost.
Start ZK instance 3 on Machine B

This process should become easier once dynamic reconfiguration is
implemented (in ZK 3.5 I believe?) because restarts won't be required.
cheers
Cam


On Tue, Nov 5, 2013 at 6:05 PM, erolagnab <trung.n.k@gmail.com> wrote:

> Thanks, I got the idea now. So is it fair to say that it is not possible to
> create ZK cluster providing some redundancy with 2 physical machines? If
> so,
> is there a way to make it happen?
>
>
>
> --
> View this message in context:
> http://zookeeper-user.578899.n2.nabble.com/Running-Zookeeper-in-2-machines-tp7579232p7579237.html
> Sent from the zookeeper-user mailing list archive at Nabble.com.
>

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