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From Flavio Junqueira <...@yahoo-inc.com>
Subject Re: Question regarding Membership Election
Date Fri, 15 Jan 2010 09:04:28 GMT
Hi Vijay, It might not have been clear in my previous message, but I  
was suggesting that with three locations you can tolerate one DC going  
down, assuming you have votes across locations. We have performed some  
preliminary tests some time ago with ZooKeeper servers across  
different locations, and it looked good. Perhaps we should think about  
running a more extensive set of tests and making them available for  

Just in case you're interested, here is perhaps an interesting example  
of how you can tolerate one location going down with three locations.  
For each location, create a group of three ZooKeeper servers. In this  
setting, we need only two votes from a remote location, independent of  
where the leader is. For example, consider the following groups and  
servers in each group:

Group 1: A B C
Group 2: D E F
Group 3: G H I

If the leader is A, then it needs to collect votes only from two  
servers in group 1, say A and B, and two votes from some other  
location, say D and E in group 2 (corresponding to location 2). To  
make it work, all you have to do is create groups in ZooKeeper, which  
enables the hierarchical quorum feature.

This example is interesting because it enables the system to mask one  
server crashing in each group/location and a whole location going  
down, and requires one fewer vote compared to absolute majority, which  
would be five servers given that we have nine total.

Hope it is useful.

On Jan 14, 2010, at 11:00 PM, Vijay wrote:

> Hi Falvio,
> Yes i am concerned about the latency between the DC's (Across  
> continents),
> We actually have 6 locations but how exactly are we going to do it  
> if we
> have the 3rd DC?
> Regards,
> </VJ>
> On Thu, Jan 14, 2010 at 1:46 PM, Flavio Junqueira <fpj@yahoo- 
> inc.com> wrote:
>> Hi Vijay, I'm just curious: why exactly you want all voting nodes  
>> in a
>> single data center? Are you concerned about latency?
>> It might not be possible in your case, but if you have a third  
>> location
>> available, you would be able to tolerate one location going down.
>> -Flavio
>> On Jan 14, 2010, at 9:12 PM, Vijay wrote:
>> Hi,
>>> I read about observers in other datacenter,
>>> My question is i dont want voting across the datacenters (So i  
>>> will use
>>> observers), at the same time when a DC goes down i dont want to  
>>> loose the
>>> cluster, whats the solution for it?
>>> I have to have 3 nodes in primary DC to accept 1 node failure. Thats
>>> fine...
>>> but what about the other DC? how many nodes and how will i make it  
>>> work?
>>> Regards,
>>> </VJ>

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