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Zookeeper >> mail # user >> Consistency in zookeeper


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Yasin 2013-02-28, 23:35
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Re: Consistency in zookeeper
Hi Yasin,

I assume you mean "linearizability" by "strict consistency".

ZooKeeper provides "sequential consistency". This is weaker than
linearizability but is still very strong, much stronger than "eventual
consistency".
In addition, all update operations are linearizable as they are
sequenced by the leader. With sequential consistency, a reader never
"goes back in time"
even if you read from a different follower every time, you'll never
see version 3 of the data after seeing version 4.

ZooKeeper also provides a sync command. If you invoke a sync command
and then a read, the read is guaranteed to see at least the last write
that
completed before the sync started. So if you always do "sync + read"
instead of just "read", you get linearizability. But you pay in
performance since
these reads will no longer be executed locally on the follower to
which you're connected - they sync is sent to the leader. That's why
ZooKeeper gives
you the option of doing a fast read that is consistent but may
retrieve a slightly old version, or a sync+read that is more
consistent but slower.

Alex

On Thu, Feb 28, 2013 at 3:35 PM, Yasin <[EMAIL PROTECTED]> wrote:
> Hello everyone,
>
> From the zookeeper website I understand that zookeeper does not provide
> strict consistency in every instance in time.
> (http://zookeeper.apache.org/doc/trunk/zookeeperProgrammers.html#ch_zkGuarantees)
> Have ever anyone considered to make zookeeper strictly consistent at
> anytime. What I mean is that any time a value is updated in zookeeper, any
> client that retrieves the value from any follower should get consistent
> result. Is it feasible to improve the zookeeper core so that zookeeper
> delivers strict consistency not the eventual consistency?
>
> Best
>
> Yasin
>
>
>
> --
> View this message in context: http://zookeeper-user.578899.n2.nabble.com/Consistency-in-zookeeper-tp7578531.html
> Sent from the zookeeper-user mailing list archive at Nabble.com.
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Rakesh R 2013-03-01, 05:32
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kishore g 2013-03-01, 17:49
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Alexander Shraer 2013-03-01, 18:19
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Flavio Junqueira 2013-03-01, 20:35
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Yasin 2013-03-01, 20:50
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Yasin 2013-03-01, 20:50
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kishore g 2013-03-01, 21:17
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Yasin 2013-03-01, 21:38
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Martin Kou 2013-03-01, 21:50
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Jordan Zimmerman 2013-03-01, 22:01
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Ted Dunning 2013-03-02, 02:03
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Rakesh R 2013-03-03, 09:51
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Ted Dunning 2013-03-02, 02:00