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From Mahadev Konar <maha...@yahoo-inc.com>
Subject Re: DR policies/HA setup in production - best practices
Date Mon, 03 Jan 2011 22:31:26 GMT
Sergei,
 I think Ted already answered you question but in case you are interested in
more details, please take a look at

http://hadoop.apache.org/zookeeper/docs/r3.2.1/zookeeperInternals.html

Thanks
mahadev


On 1/3/11 1:43 PM, "Ted Dunning" <ted.dunning@gmail.com> wrote:

> Actually, ZK is very good in this regard.
> 
> The lifetime of a single leader is denoted by an epoch number.  Transactions
> are identified by an epoch and a sequence number assigned by the leader.
>  Since there is only one leader and because all transactions are executed
> serially, this
> combination of epoch and transaction id uniquely specifies a transaction and
> provides a complete ordering.
> 
> As transactions are committed, members of the committing quorum record the
> latest epoch and transaction.
> 
> When you restart a cluster, the members of the cluster negotiate to
> determine who has the latest transaction and then start from there.  As
> such, it is probably a good idea to backup more than just one log+snapshot
> so that you have a better chance of having a later copy.
> 
> On Mon, Jan 3, 2011 at 12:58 PM, Sergei Babovich
> <sbabovich@demandware.com>wrote:
> 
>> It is also understood about DR strategy. What is the mechanism for ZK to
>> resolve conflicts in such case? Let's say we have a primitive backup
>> strategy of shipping logs every hour. In theory it means (assuming the worst
>> case) that on DR site all servers will have snapshots of the data made at
>> different point in time. When I bring the DR cluster up what is a protocol
>> of resolving inconsistencies? That was a reason of my question - it felt
>> (may be naively) that recovering by replicating from the single node data
>> (snapshot+log) would be safer and more consistent approach - it is easier to
>> make guaranties about result.
>> 
>> 
> 


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