Re: Cross DC SolrCloud anti-patterns in presentation shalinmangar/cross-datacenter-replication-in-apache-solr-6

[Arcadius Ahouansou <ar...@menelic.com>]

Hello Shawn.

Thank you very much for the comment.

On 24 June 2017 at 16:14, Shawn Heisey <ap...@elyograg.org> wrote:

> On 6/24/2017 2:14 AM, Arcadius Ahouansou wrote:
> > Interpretation 1:
>
> ZooKeeper doesn't *need* an odd number of servers, but there's no
> benefit to an even number.  If you have 5 servers, two can go down.  If
> you have 6 servers, you can still only lose two, so you might as well
> just run 5.  You'd have fewer possible points of failure, less power
> usage, and less bandwidth usage.
>
>
About Slide 8 and the odd/even number of nodes...
what I meant is that on Slide 8, if you loose DC1, then your cluster will
not be able to recover after DC1 comes back as there will be no clear
majority
and you will have:
-  3 ZK nodes with up-to-date data (that is DC2+DC3) and
-  3 ZK nodes with out-of-date data (DC1).

But, if you had only 2 ZK nodes in DC1, then you could afford to loose one
of either DC1, or DC2 or DC3 and the cluster will be able to recover and be
OK


Thank you very much.


Arcadius

-- 
Arcadius Ahouansou
Menelic Ltd | Applied Knowledge Is Power
Office : +441444702101
Mobile: +447908761999
Web: www.menelic.com
---

Re: Cross DC SolrCloud anti-patterns in presentation shalinmangar/cross-datacenter-replication-in-apache-solr-6

[Erick Erickson <er...@gmail.com>]

bq: if you loose DC1, then your cluster will not be able to recover
after DC1 comes back as there will be no clear majority

When ZK loses majority, no indexing takes place. So in the case where
you have 3 nodes in DC1 and 3 nodes in DC2, _neither_ of them would
allow updates if the connection was cut for any reason since updates
require 4 live ZK servers in this scenario to be available ((6/2)+1).
So when the connection was restored, there'd be nothing to reconcile
and Solr should recover just fine.

The whole ZK majority thing is about data consistency. Since querying
doesn't change the index at all there's no consistency problem here to
reconcile after the connection is restored. And since quorum was lost,
no updates are allowed.

Best,
Erick

On Mon, Jul 10, 2017 at 5:08 PM, Arcadius Ahouansou
<ar...@menelic.com> wrote:
> Hello Shawn.
>
> Thank you very much for the comment.
>
> On 24 June 2017 at 16:14, Shawn Heisey <ap...@elyograg.org> wrote:
>
>> On 6/24/2017 2:14 AM, Arcadius Ahouansou wrote:
>> > Interpretation 1:
>>
>> ZooKeeper doesn't *need* an odd number of servers, but there's no
>> benefit to an even number.  If you have 5 servers, two can go down.  If
>> you have 6 servers, you can still only lose two, so you might as well
>> just run 5.  You'd have fewer possible points of failure, less power
>> usage, and less bandwidth usage.
>>
>>
> About Slide 8 and the odd/even number of nodes...
> what I meant is that on Slide 8, if you loose DC1, then your cluster will
> not be able to recover after DC1 comes back as there will be no clear
> majority
> and you will have:
> -  3 ZK nodes with up-to-date data (that is DC2+DC3) and
> -  3 ZK nodes with out-of-date data (DC1).
>
> But, if you had only 2 ZK nodes in DC1, then you could afford to loose one
> of either DC1, or DC2 or DC3 and the cluster will be able to recover and be
> OK
>
>
> Thank you very much.
>
>
> Arcadius
>
> --
> Arcadius Ahouansou
> Menelic Ltd | Applied Knowledge Is Power
> Office : +441444702101
> Mobile: +447908761999
> Web: www.menelic.com
> ---