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Date: Tue, 8 Mar 2016 17:18:58 -0800
Message-ID: 
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Subject: Re: Multi DC ( DC-1 and DC-2) zookeeper setup
From: s influxdb <elastic.l.k@gmail.com>
To: user@zookeeper.apache.org
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I am referring to a set up that has different clusters
for example 3 zk cluster
cluster ABC  DC1 { node 1, node 2 } DC 2 { node 3 , node 4 } DC 3 { node 5}
cluster DEF  DC2 { node 6, node 7 } DC 1 { node 8 , node 9 } DC 3 { node 10}
cluster GHI  DC3 { node 11, node 12 } DC 2 { node 13 , node 14 } DC 1 {
node 15}

This survives any single DC being unavailable.

My question was how is the data kept in sync among the  3 different zk
clusters. for example between cluster ABC and DEF.
and how is the client failing over to DEF when ABC is unavailable


On Tue, Mar 8, 2016 at 4:10 PM, Shawn Heisey <apache@elyograg.org> wrote:

> On 3/8/2016 3:40 PM, s influxdb wrote:
> > How does the client failover to the DC2 if DC1 is down ? Does the
> services
> > registered on DC1 for example with ephemeral nodes have to re-register
> with
> > DC2 ?
>
> Even though Flavio and Camille have both said this, I'm not sure whether
> the posters on this thread are hearing it:
>
> If you only have two datacenters, you cannot set up a reliable zookeeper
> ensemble.  It's simply not possible.  There are NO combinations of
> servers that will achieve fault tolerance with only two datacenters.
>
> The reason this won't work is the same reason that you cannot set up a
> reliable ensemble with only two servers.  If either data center goes
> down, half of your ZK nodes will be gone, and neither data center will
> have enough nodes to achieve quorum.
>
> When you have three datacenters that are all capable of directly
> reaching each other, you only need one ZK node in each location.  If any
> single DC goes down, the other two will be able to keep the ensemble
> running.
>
> Data is replicated among the DCs in exactly the same way that it is if
> all the servers are in one place.  I don't know enough about internal ZK
> operation to comment further.
>
> =============
>
> Some TL;DR information to follow:
>
> If you want to be able to take a node down for maintenance in a multi-DC
> situation and *still* survive an entire DC going down, you need three
> nodes in each of three data centers -- nine total.  This ensemble is
> able to survive any four servers going down, so you can take down a node
> in one DC for maintenance, and if one of the other DCs fails entirely,
> there will be five functioning servers that can maintain quorum.
>
> Detailed information for the specific situation outlined by Kaushal:
>
> DC-1 1 Leader 2 Followers
> DC-2 1 Follower 2 Observers.
>
> A six-node ensemble requires at least operational four nodes to maintain
> quorum.  If either of those data centers fails, there are only three
> nodes left, which is not enough.
>
> Thanks,
> Shawn
>
>

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