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From Enis Söztutar <e...@hortonworks.com>
Subject Re: Hadoop Summit EU
Date Mon, 07 Apr 2014 18:36:04 GMT
Ops sorry this was intented for internal lists. Apologies for any


On Monday, April 7, 2014, Enis Söztutar <enis@hortonworks.com> wrote:

> Me and Devaraj attended their talk on their solution for paxos based
> namenode and HBase replication.
> They have two solutions, one for single datacenter, and the other multi DC
> geo replication.
> For the namenode, there is a wrapper, called ConsistencyNode, that
> basically gets the requests, replicate it via their consistency protocol to
> other CNodes within the DC (paxos based) in the edit log. If the proposal
> for this is accepted, the changes are made durable. However, from my
> understanding, on the read side the client chooses only one replica to
> read. The client decides to connect to one of the replica namenodes, which
> means that it is not doing a paxos read. I think they also wrapped the
> client, so that if it gets a FileNotFoundException or something similar, it
> will retry on a different server. Also they track the last seen proposal id
> as a transaction id for this as well from my understanding (so
> read-what-you-write consistency maybe?). The full details of the
> consistency was not clear to me from the presentation.
> For their multi-DC replication, they are doing a similar thing, but the
> data replication is not handled by paxos, only the namenode metadata. For
> each datacenter, they have a target replication factor (can be set
> differently for each DC, like 0 because of regulatory reasons). The
> metadata of NN is replicated via a similar mechanism. The data replication
> is async to the metadata replication though. When a block is finalized, the
> CNode quorum on that particular DC, schedules a remote copy to one of the
> datacenters. That copy job, copies the block with directly writing the
> block from the datanode to a remote datanode. Then that remote DC block is
> replicated to the target replication by that DC's CNode quorum. When the
> target is reached, that DC will create another proposal about the data
> replication being complete. So the state machine probably contains where
> each data is replicated, but they were still mentioning the client getting
> DataNotReplicatedException or something.
> Their work on HBase is still WIP. I do not remember much details on the
> protocol, except it uses the same replication protocol (their "patented"
> paxos based replication).
> Of course the devil is in the details. I did not get that from the
> presentation.
> As a side note, Doug when asked, was saying that they are cooking
> something for backups, so maybe their "secret project" also contains
> multi-DC consistent state?
> Enis
> On Sat, Apr 5, 2014 at 1:55 AM, Ted Yu <yuzhihong@gmail.com<javascript:_e(%7B%7D,'cvml','yuzhihong@gmail.com');>
> > wrote:
>> Enis:
>> There was a talk by Konstantin Boudnik<http://hadoopsummit.org/amsterdam/speakers/#konstantin-boudnik>
>> .
>> Any interesting material from his presentation ?
>> Cheers

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