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From Flavio Junqueira <...@yahoo-inc.com>
Subject Re: Achieving quorum with only half of the nodes
Date Thu, 15 Jul 2010 16:26:36 GMT
Your EC2 suggestion sounds reasonable. If your deployment is able to  
form a local quorum most of the time, then you would be able to get a  
quorum of acks most of the time.

One concern is that the EC2 replica might lag behind badly, which may  
force the leader to either slow down or to drop the connection to the  
EC2 follower, assuming that EC2 server is not the leader itself.

It might not be a possibility for you, but Ideally, you could have  
three power sources, and have three sets of servers. We could then  
tolerate the failure of one power source with the mechanisms we have  
currently implemented.

-Flavio

On Jul 14, 2010, at 11:16 PM, Sergei Babovich wrote:

> Thanks, Flavio,
> Yep... I see. This is a problem. Any better idea?
> As an alternative option we could probably consider running single ZK
> node on EC2 - only in order to handle this specific case. Does it make
> sense to you? Is it feasible? Would it result in considerable
> performance impact due to network latency? I hope that at least in
> theory since quorum can be reached without ack from EC2 node  
> performance
> impact might be manageable.
>
> Regards,
> Sergei
>
> On 07/14/2010 04:52 PM, Flavio Junqueira wrote:
>> Hi Sergei, I'm not sure what the implementation of QuorumVerifier you
>> have in mind would look like to make your setting work. Even if you
>> don't have partitions, variation in message delays can cause
>> inconsistencies in your ZooKeeper cluster. Keep in mind that we make
>> the assumption that quorums intersect.
>>
>> -Flavio
>>
>> On Jul 14, 2010, at 9:43 PM, Sergei Babovich wrote:
>>
>>> Hi,
>>> We are currently evaluating use of ZK in our infrastructure. In our
>>> setup we have a set of servers running from two different power  
>>> feeds.
>>> If one power feed goes away so does half of the servers. This makes
>>> problematic to configure ZK ensemble that would tolerate such  
>>> outage.
>>> The network partitioning is not an issue in our case. The only  
>>> solution
>>> I come up with so far is to provide custom QuorumVerifier that  
>>> will add
>>> a little premium in case if all servers in the quorum set are from  
>>> the
>>> same group. Basically if we have only half of votes but all of them
>>> belong to the same group then we decide to have a quorum.
>>> Any ideas or better solutions are very appreciated. Sorry if this  
>>> has
>>> been already discussed/answered.
>>>
>>> Regards,
>>> Sergei
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>>
>> *flavio*
>> *junqueira*
>>
>> research scientist
>>
>> fpj@yahoo-inc.com <mailto:fpj@yahoo-inc.com>
>> direct +34 93-183-8828
>>
>> avinguda diagonal 177, 8th floor, barcelona, 08018, es
>> phone (408) 349 3300 fax (408) 349 3301
>>
>>
>
>
>
> This e-mail message and all attachments transmitted with it may  
> contain privileged and/or confidential information intended solely  
> for the use of the addressee(s). If the reader of this message is  
> not the intended recipient, you are hereby notified that any  
> reading, dissemination, distribution, copying, forwarding or other  
> use of this message or its attachments is strictly prohibited. If  
> you have received this message in error, please notify the sender  
> immediately and delete this message, all attachments and all copies  
> and backups thereof.

flavio
junqueira

research scientist

fpj@yahoo-inc.com
direct +34 93-183-8828

avinguda diagonal 177, 8th floor, barcelona, 08018, es
phone (408) 349 3300    fax (408) 349 3301




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