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From Ketan Gangatirkar <ke...@indeed.com>
Subject Re: observers in occasionally disconnected data centers
Date Wed, 04 May 2011 01:42:11 GMT
I have updated Sergey's patch to:

* apply to current trunk
* incorporate one trivial output change he made to StatCommand in
* change log4j references to slf4j

I have successfully run ant releaseaudit on the result.  The updated
patch is now attached to the issue:


I do *not* make any claim to have understood the contents of this
patch; all I did was synch everything and fix the obvious log4j/slf4j
change.  Now what?

On Tue, May 3, 2011 at 5:46 PM, Patrick Hunt <phunt@apache.org> wrote:
> The core tests failed on last hudson, I just kicked off a patch build,
> seems recent changes (logging?) have caused the patch to stop
> applying:
> https://hudson.apache.org/hudson/view/S-Z/view/ZooKeeper/job/PreCommit-ZOOKEEPER-Build/238/console
> Ketan would you like to try updating the patch and resubmit?
> Patrick
> On Tue, May 3, 2011 at 3:31 PM, Ketan Gangatirkar <ketan@indeed.com> wrote:
>> Thanks, Mahadev.  I had seen ZOOKEEPER-892 but not ZOOKEEPER-784.  The
>> latter may be what we need.
>> I read the comments attached to that issue.  The most recent comment
>> was a Hudson CI message indicating that the tests against the patch
>> failed.  I was not able to find out more as it appears that the
>> configuration of the Apache Hudson has changed.  It appears that the
>> patch was approved but not merged into trunk, and it's now in limbo.
>> What is necessary to get that feature into the next release?  I may be
>> able to assist, depending on what's involved.  Thank you.
>> On Tue, May 3, 2011 at 4:17 PM, Mahadev Konar <mahadev@apache.org> wrote:
>>> Hi Ketan,
>>>  You are correct that observers need connection to quorum as well.
>>> There have been quite a few discussions on multi colo replication and
>>> read only mode of ZooKeeper.
>>> Here are the jiras for those:
>>> https://issues.apache.org/jira/browse/ZOOKEEPER-784
>>> and
>>> https://issues.apache.org/jira/browse/ZOOKEEPER-892
>>> These have been mostly targeted at exactly a use case like yours.
>>> Please take a look and them and feel free to contribute/comment on the
>>> jiras.
>>> --
>>> thanks
>>> mahadev
>>> @mahadevkonar
>>> On Tue, May 3, 2011 at 2:07 PM, Ketan Gangatirkar <ketan@indeed.com> wrote:
>>>> Hi.  We're considering ZooKeeper for coordinating operations across
>>>> multiple data centers.  These data centers will occasionally be
>>>> disconnected.  We were planning on using observers in remote data
>>>> centers.  Our applications can survive being unable to *write* to
>>>> ZooKeeper, but they do need to be able to read from it, even if the
>>>> data were stale.
>>>> On further examination, it looks like observers must always be
>>>> connected to the quorum to function at all.  Is this correct?  Does
>>>> anyone have suggestions for how to work around this problem?  The
>>>> first thing that comes to mind is duplicating the required data in
>>>> some other local data store and falling back on that when the DC
>>>> becomes disconnected.  I imagine the disadvantages of that are obvious
>>>> to everyone.  I hope someone can share some great idea that allows me
>>>> to avoid that miserable fate.  Thanks.
>>>> --
>>>> Ketan Gangatirkar
>>>> ketan@indeed.com
>> --
>> Ketan Gangatirkar
>> ketan@indeed.com
>> Perishable Developer

Ketan Gangatirkar
Perishable Developer

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