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From "Jack Krupansky" <j...@basetechnology.com>
Subject Re: Minimum HA Setup with SolrCloud
Date Thu, 06 Dec 2012 22:55:18 GMT
I trust that you have the right answer, Mark, but maybe I'm just struggling 
to parse this statement: "the remaining two machines do not constitute a 
majority."

If you start with 3 zk and lose one, you have an ensemble that does not 
"constitute a majority".

So, the question is what can or can't you do once your ensemble no longer 
has a majority? Why does "majority" matter if you can run just fine? I mean, 
there must be some hard-core downside, other than that you can't lose any 
more nodes.

-- Jack Krupansky

-----Original Message----- 
From: Mark Miller
Sent: Thursday, December 06, 2012 5:49 PM
To: solr-user@lucene.apache.org
Subject: Re: Minimum HA Setup with SolrCloud

3 is the minimum if you want to allow a node to go down.

1 is the minimum if you want the thing to work at all - but if the 1 goes 
down, ZooKeeper may stop working…

- Mark

On Dec 6, 2012, at 2:21 PM, Jack Krupansky <jack@basetechnology.com> wrote:

> Rewind.
>
> If 1 is the minimum, what is the 3 "minimum" all about?
>
> The zk web page does say "Three ZooKeeper servers is the minimum 
> recommended size for an ensemble, and we also recommend that they run on 
> separate machines" - but it does say "recommended".
>
> But back to the original question - it sounds as if 4 nodes would the 
> recommended minimum number of nodes if you want to tolerate one machine 
> going down and maintaining that 3 zookeeper recommended minimum ensemble.
>
> From the zookeeper web page:
>
> "For reliable ZooKeeper service, you should deploy ZooKeeper in a cluster 
> known as an ensemble. As long as a majority of the ensemble are up, the 
> service will be available. Because Zookeeper requires a majority, it is 
> best to use an odd number of machines. For example, with four machines 
> ZooKeeper can only handle the failure of a single machine; if two machines 
> fail, the remaining two machines do not constitute a majority. However, 
> with five machines ZooKeeper can handle the failure of two machines."
>
> See:
> http://zookeeper.apache.org/doc/r3.1.2/zookeeperAdmin.html
>
> -- Jack Krupansky
>
> -----Original Message----- From: Otis Gospodnetic
> Sent: Thursday, December 06, 2012 4:39 PM
> To: solr-user@lucene.apache.org
> Subject: Re: Minimum HA Setup with SolrCloud
>
> 1 is the minimum :)
> 2 makes no sense.
> 3 must be the most common number in the zoo.
>
> Otis
> --
> Performance Monitoring - http://sematext.com/spm/index.html
> Search Analytics - http://sematext.com/search-analytics/index.html
>
>
>
>
> On Thu, Dec 6, 2012 at 9:46 AM, Jack Krupansky 
> <jack@basetechnology.com>wrote:
>
>> "but if those same machines are running zookeeper, you need 3."
>>
>> And one of those 3 can go down? I thought 3 was the minimum number of
>> zookeepers.
>>
>> -- Jack Krupansky
>>
>> -----Original Message----- From: Mark Miller
>> Sent: Thursday, December 06, 2012 9:30 AM
>> To: solr-user@lucene.apache.org
>> Subject: Re: Minimum HA Setup with SolrCloud
>>
>>
>> It depends on if you are running embedded zk or an external zk ensemble.
>>
>> One leader and a replica is all you need for Solr to allow on machine to
>> go down - but if those same machines are running zookeeper, you need 3.
>>
>> You could also run zookeeper on one external machine and then it would be
>> fine if you lost one solr node - but if you the one external zk node went
>> down you would lose the ability to do updates until it was brought back 
>> up.
>>
>> - Mark
>>
>> On Dec 6, 2012, at 2:46 AM, Thomas Heigl <thomas@umschalt.com> wrote:
>>
>> Hey all,
>>>
>>> I'm in the process of migrating a single Solr 4.0 instace to a SolrCloud
>>> setup for availability reasons.
>>>
>>> After studying the wiki page for SolrCloud I'm not sure what the 
>>> absolute
>>> minimum setup is that would allow for one machine to go down.
>>>
>>> Would it be enough to have one shard with one leader and one replica?
>>> Could
>>> this setup tolerate one of the instances going down? Or do I need three
>>> instance because Zookeeper needs a quorum of instances?
>>>
>>> Cheers,
>>>
>>> Thomas
>>>
>>
> 

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